Chapter 2: Water Levels
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| Half tide at Sapelo Island, GA. |
Surviving Thriving on the ICW
2. Forces that Change the Water Level on the ICW
Introduction
The ICW is infamous for its shallow waters, shifting channels, and low bridge clearances. Anything that changes the water level affects channel depths and bridge clearances, and thus the ability of boats to navigate the waterway. For many of our boats, a change in water level of less than a foot can make all the difference in the world.
There are four forces on the ICW that can change the water level: lunar tides, wind, rainfall, and relative sea level rise. These factors differ in their magnitude, duration, and predictability. In some places, like Florida’s Mosquito & Indian River Lagoons and North Carolina’s Currituck, Albemarle & Pamlico Sounds, lunar tides are relatively unimportant, accounting for just a few inches of change in water level. But it’s not uncommon for the wind to cause changes in water level of 2 to 3 feet in these areas. During strong storms, the change in water level can be even greater. In contrast, on the stretch of the AICW that passes through Georgia, the tidal range can be as much as 9 feet. The wind also changes water levels in Georgia, but it’s just not as noticeable because the large tides are so dominant. Recognizing the conditions under which these changes in water level occur can help you avoid all kinds of trouble.
Tides
When we are talking about tides, we're only referring to the gravitational effects of the moon (primarily) and sun (secondarily) on the depth and movement of the water. The tide is not the only factor that changes water depth. There are other factors besides tides that influence water depth and water motion, but we’ll consider those issues later in the chapter.
Tides are caused by the gravitational pull of the moon and sun. In simple terms, the tide is a wave-like bulge in the ocean that encircles the Earth like a hula hoop around a beach ball. This hula hoop-shaped wave continually rotates around the planet, tracking the moon’s orbit around the Earth. High tide takes place as the crest of this wave passes your location and low tide happens when the trough passes you. It takes roughly 24 hours and 50 minutes for this circular wave to make one full rotation around the Earth. So at any particular location (with exceptions that aren’t relevant to the AICW), there are generally two high tides each day, spaced out by 12 hours and 25 minutes. Similarly, there are two low tides each day, also every 12 hours and 25 minutes. If high tide at Beaufort Inlet, NC, occurs at 12:00 PM on Tuesday, then the next low tide will be on Tuesday at 6:12 PM and the next high tide will be at 12:25 AM on Wednesday. And if high tide was at noon on Tuesday, then the tide will be high on Wednesday afternoon at 12:50 PM. It also means that one week later, on the following Tuesday, the tide at Beaufort Inlet will be low at just a few minutes before noon. When planning the legs of your ICW cruise, it’s helpful to know that the tides “flip” each week. That is, if the tide was low during the mid-morning today, it will be high in the mid-morning a week from now.
How Tides Affect the ICW
When it comes to tides, there are two things that you have to remember:
Tides cause the water level to go up and down, switching roughly every 6 hours and 12 minutes.
Tides do not just cause the water level to move up and down; the water also sloshes in and out of the estuaries, causing tremendous currents in some areas, during certain stages of the tide. In general, these currents also reverse direction every 6 hours and 12 minutes (but see details below) and they reach their peak velocity halfway between the low and high tides.
The changes in water level have obvious implications for grounding in shallow water and for vertical clearance at bridges. However, tidal currents, and the difficulties they cause for maneuvering a boat, are sometimes overlooked by novice boaters or boaters whose previous experience was on lakes.
Many people picture the AICW as one long, dredged canal from Norfolk to Miami. In reality, the ICW mostly follows natural rivers and tidal creeks in brackish estuaries that are separated from the ocean by barrier islands. The ICW simply connects these formerly isolated river systems with man-made canals or “cuts” between them. So the ICW mainly follows the estuaries, sandwiched between the mainland and the barrier islands, and crosses inlets perpendicularly. (It is worth mentioning here that there are parts of the AICW in South Carolina and Virginia that head quite far inland, through freshwater).
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The ICW at Lockwood’s Folly Inlet, SC (MM 321). The Atlantic Ocean is at the bottom of the image and the mainland is at the top. The approximate route of the ICW is shown by the black dashed line. The color ramp indicates depths (blue is deep, red is shallow). Source: Waterway Guides Explorer (showing Aqua Map & US Army Corps of Engineers Depth Survey, https://www.waterwayguide.com/explorer?latitude=33.91526193338409&longitude=-78.23668857151733&zoom=13.74321049882461&mode=navAlert) |
For ICW cruisers, the important point to remember is that the tide is an ocean phenomenon; the incoming tide comes from the ocean and the outgoing tide returns to the ocean via inlets. As the crest of this tidal bulge or wave passes by an inlet that connects an estuary to the ocean, the water in the ocean just outside the inlet will be at a higher elevation than the water inside the adjacent estuary. Since gravity causes water to flow downhill, it flows from the ocean into the estuary via the inlet (the incoming tide or flood tide). As the tide’s wave keeps moving past the inlet and the trough approaches, the water level in the estuary is now higher than that in the adjacent ocean, so the water flows back out to the ocean through the inlet (the outgoing tide or ebb tide). As you are traveling on the ICW, the current from an incoming tide will be coming from an inlet (usually the inlet that is closest to you) and the current from an outgoing tide will be traveling toward an inlet (again, usually the closest one).
The timing and magnitude of the tides at two adjacent inlets at the opposite ends of a barrier island will be very similar; usually no more than a few minutes’ difference. If the tide is coming in at one inlet, it will also be coming in at the neighboring inlets, too. The incoming tidal currents from two adjacent inlets will meet and oppose each other in the estuary behind the central part of the barrier island, roughly halfway between the two adjacent inlets. This region behind the central portion of a barrier island is called a tidal node or tidal convergence zone. The tidal currents are generally slower as you approach the tidal convergence zone and grow progressively stronger the closer you are to the mouth of an inlet.
Jekyll Island in Georgia, which is separated from the mainland by the Jekyll Creek section of the ICW. St. Simons Inlet lies to the north of Jekyll Island and St. Andrews Inlet to the south. The incoming tidal current flows into both of these inlets from the ocean and into the opposite ends of Jekyll Creek, converging in the creek behind the central portion of the island (red arrows). Currents in this tidal node, or zone of convergence, tend to be relatively weak, causing shoaling as sediments suspended by the moving water are deposited on the seafloor. (Source: Waterway Guide Viewer with Aqua Map and USACE depth survey, https://www.waterwayguide.com/explorer?latitude=31.06444657856106& longitude=-81.36537263867964&zoom=11.366678502717622&mode=bridge)
Jekyll Creek, showing incoming tidal currents converging (red arrows) and the resulting shoaling in this zone of convergence, between MM 683 and 684.5. (Source: Waterway Guide Viewer with Aqua Map and USACE depth survey, https://www.waterwayguide.com/explorer?latitude=31.061649888793312& longitude=-81.39722441537296&zoom=13.299920555908667&mode=bridge)
Although the timing of the tides at the mouths of two inlets that are 10 or 20 miles apart are usually pretty similar, things get complicated as you move inland from the ocean, up the estuary through the network of tidal creeks. There can be large differences in the timing of the tides across a distance of just a few miles between the mouth of an inlet and the upper estuary. A difference of an hour or two is not uncommon for locations that are just a few miles apart, as the crow flies. The tides inside the estuary lag behind those at the mouth of the inlet; the further up the estuary you go, the longer the time lag is, compared to the tides at the mouth of the inlet. For example, the tide at the Duke University Marine Laboratory on Pivers Island, near the town of Beaufort, NC, lags behind the tide at the mouth of Beaufort Inlet by roughly 24 minutes, and the tide at the Core Creek Bridge (MM 196) lags behind the tide at Beaufort Inlet by 71 minutes. Following the navigation channels, the distance from Beaufort Inlet is 2.3 statute miles at the Duke Marine Lab and 10.8 statute miles at the Core Creek Bridge. Thus, by heading inland a distance of less than 11 miles, you will experience a difference in the timing of the tides of 1 hour and 11 minutes.
Tide at the town of Swansboro, NC (East Corbett Ave Bridge), which is on the mainland side of the estuary, lags behind the tide at the mouth of nearby Bogue Inlet, NC, by nearly 2 hours, even though these locations are only about 2.8 miles apart, as the crow flies (3.7 miles following the channel through the estuary). In contrast, the tides at the mouths of Beaufort Inlet, Bogue Inlet, and Masonboro Inlet are all within about 10 minutes of each other, even though Masonboro and Beaufort Inlets are separated by almost 75 miles (Bogue Inlet lies between the other two). These differences in timing between the outer inlets and upper estuaries are important if you are trying to predict the depth at a particular location and time. Using predictions for the closest tide station may not be appropriate. Instead, you need to find a tide station that is a similar distance inland from the mouth of the inlet in river miles, not as the crow flies. Think about the total distance that a molecule of water would have to travel to get from the ocean to your location within the estuary. The longer that distance is, the greater the difference in time (Hint: Most chart plotting programs that provide tide information default to the closest tide station as the crow flies. When using your chart plotter to look up the timing of the tides, make sure you pay attention to the tide station from which the data originate.)
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| Beaufort, Bogue and Masonboro Inlets in southern North Carolina. The red dots indicate the locations of tide stations. Beaufort and Masonboro Inlets are 74 statute miles apart “as the crow flies” and 81 statute miles apart, along the ICW. On average, tides at Bogue Inlet occur approximately 5 minutes after those at Beaufort Inlet and tides at Masonboro Inlet occur 6 minutes before tides at Beaufort Inlet. |
Four tide stations in close proximity to Beaufort Inlet (same stations as those shown by the dots in the previous figure). These stations are: 1. Mouth of Beaufort Inlet, 2. Morehead City (+18 minutes from Beaufort Inlet), 3. Newport River (+59 minutes), and 4. Core Creek Bridge (+71 minutes). The approximate route of the ICW is shown by the yellow dashed line. Stations 1 and 4 are separated by 10.8 statute miles along the channel and 9.2 miles as the crow flies. Tidal range at Core Creek Bridge (4) is approximately 1.4 feet less than the range at Beaufort Inlet.
As you travel along the ICW, the direction and speed of the current will change frequently, both because the phase of the tide changes over time and because your location relative to the inlets changes as you move. The ICW parallels the coastline and crosses countless inlets. If you are traveling along the ICW and approach an inlet on an incoming tide, the current will be coming from the ocean via the inlet and will be flowing against you. As you reach the inlet, the direction of the current will change quickly and begin pushing you sideways, toward the mainland side of the channel. Then as you pass that inlet and start heading away from it, the current direction will abruptly flip and flow with you, pushing you faster. The opposite would be true on an outgoing tide: you will have the current behind you as you approach the inlet; at the inlet, the current will be dragging you sideways toward the ocean; and once you’ve passed the inlet, you will be fighting against the current. The tidal currents near the inlets are strong and they will change direction rapidly as you cross the inlet, heading north or south in the ICW.
A similar change in current direction will happen as you pass the tidal convergence zone near the midway point between two adjacent inlets, but in this case the change is more gradual. If the incoming tide is with you after you pass an inlet, the strength of that favorable current will progressively decrease until you reach the tidal convergence zone. There will be no current at the convergence (because the water is flowing toward that spot from both north and south) but as your boat continues to move along the ICW, the current will slowly increase in the opposite direction, accelerating as you approach the next inlet.
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| Current directions on the incoming tide at an inlet (in this case, Lockwood’s Folly, MM 321). The Atlantic Ocean is at the bottom of the image and the mainland is at the top. The approximate route of the ICW is shown as the dashed yellow line. Source: Waterway Guides Explorer (showing Aqua Map, https://www.waterwayguide.com/explorer?latitude=33.915694398636504& longitude=-78.2316362297578&zoom=13.966023119790773&mode=bridge) |
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| Current directions on the outgoing tide at an inlet (in this case, Lockwood’s Folly, MM 321). The Atlantic Ocean is at the bottom of the image and the mainland is at the top. The approximate route of the ICW is shown as the dashed yellow line. Source: Waterway Guides Explorer (showing Aqua Map, https://www.waterwayguide.com/explorer?latitude=33.915694398636504& longitude=-78.2316362297578&zoom=13.966023119790773&mode=bridge) |
You can think of the tides sloshing in and out of an estuary as being similar to the air being inhaled and exhaled in and out of your lungs. The incoming tidal current starts out slow at low tide, reaches a peak in velocity at half-tide, then gradually slows, eventually coming to a stop at high tide and then reversing direction. The important point to remember about this inhaling and exhaling of the tides is that the rate of change in the water depth, and the speed of the current, are both low during the two 2-hour periods centered on high and low tides. So, in general, the current is very slow from one hour before the tide to one hour after the tide (both high tide and low tide). This is called slack tide (high slack and low slack). The water level also changes very little during this slack tide period. The speed of the tidal current reaches its maximum at half-tide, roughly three hours from both high and low tide. This is called “full flood” (incoming tide) and “full ebb” (outgoing tide). The water depth also changes most rapidly near half-tide. If you are timing your passage of a certain segment of the ICW to coincide with slack currents, plan to arrive within the two hour window, beginning an hour before high tide or low tide (but see below).
Tidal fluctuations during March 7, 2024, in Doboy Sound at Sapelo Island, GA (near MM 649). Tidal heights are given in feet, relative to mean lower low water, which is designated as 0.0 feet. The blue line in the graph indicates the height of the water at all times, relative to mean lower low tide. During the high tide at 5:28 AM, the height of the water was 7.14 feet above the level at mean lower low water. During the low tide at 11:58 AM, the height of the water was 0.09 feet lower than the level at mean lower low water. Note that the water level changes relatively little within the hour before and after high tide and low tide (where the slope of the blue line is closest to being horizontal). These periods correspond to the slack tides, the time periods when the tidal currents are weakest. In contrast, the water level changes most rapidly during the 3rd and 4th hours following a tide, and this is where the slope of the blue line is steepest. This period corresponds with full flood and full ebb, when the tidal currents are strongest. (Source: https://tidesandcurrents.noaa.gov/tide_predictions.html)
At this point, it is important to mention that tide and current are not exactly the same. Although we often make the assumption that tidal currents change direction precisely when high tide and low tide are reached at a particular location, this is not always the case. If you stand on the bank of a tidal river and look out at the water, the direction in which the current will flow (right to left or left to right) depends entirely on whether the water level to your left is higher or lower than the level to your right. Water always runs downhill. In the river estuaries, it is common for the outgoing current flow to persist for a longer period of time than the incoming flow because the current continues to ebb for a while after low tide is reached. Even though the tide has turned from ebbing to flooding, the water upstream is still higher than that downstream for a while after low tide has passed. (In other words, you are in the intertidal zone, which is the vertical zone between low and high tide. So at low tide, this location is above the level of the ocean’s surface.) The current will continue to ebb until the incoming tide raises the level of the ocean’s surface to the elevation of the river at your location.
So, depending on where you are positioned vertically in the intertidal zone, the change in current direction may not coincide perfectly with low tide; the current may continue to ebb for some period after low tide. But in any given location, the lags between high tide and high slack and between low tide and low slack should be fairly consistent. For example, at a particular location, the current may begin to ebb immediately after high tide, but it may not start to flood until an hour past low tide. This phenomenon is most noticeable on the larger rivers that cross the ICW, such as the St. John’s, Savannah, and Cape Fear. The higher your elevation is in the intertidal zone (i.e., the further upstream you are from the ocean), the longer the time lag between low tide and the time when the incoming current begins.
In many estuaries, the current on the outgoing tide is stronger than that on the incoming tide. The current of the incoming tide opposes the outward flow of river water, resulting in a net flow that is relatively slow. In contrast, on the outgoing tide, the tidal currents and river flow are both going in the same direction, making for very strong currents. On a side trip from the ICW we visited Darien, GA, in the Altamaha River Delta. The placid trip up the river on the incoming tide was a stark contrast to the ebb tide we experienced a few hours later once we were on the dock. We realized that if we hadn’t made our trip upstream on the flood tide, there would have been no way to motor against the ebb. Strong outgoing tidal currents opposing an onshore breeze can also create dangerous sea conditions in the inlets, with very steep standing waves that are close together (see Chapter 6).
Monthly, annual, and even longer cycles affect the tidal range. At any particular location, tidal range and tidal current velocity are greater for several days around the new and full moons, especially during fall. These “spring tides” occur every two weeks, during the full and new moon. (“Spring tide” refers to periods where the high tides are higher than average and low tides are lower than average, and have nothing to do with the season.) This is because during the new and full moons, the sun, Earth, and moon are aligned; this is called syzygy. Conversely, during the half moon, these three celestial objects form a right angle to one another. Thus, during the full and new moons, gravity from the sun and moon pulls in the same direction, whereas during the half moon, the pull from these two celestial bodies is from different directions. You can expect stronger currents, higher high tides, and lower low tides during the full and new moons. And you will probably notice these “bigger” tides for about 3 or 4 days before and after the full and new moon. For a few days before and after the half moons we see “neap tides,” when tidal ranges and the speeds of tidal currents are a bit lower than average. The half moon (and neap tides) also happen every two weeks.
Tidal range varies along the East Coast. As you move south along the oceanfront beaches from Cape Hatteras, NC (~3 feet), the tidal range increases, reaching a maximum (~9 feet) near Sapelo Island, GA. Tidal range then diminishes as you continue moving further south (~5 feet in St. Augustine, ~3.5 feet at Canaveral, and ~2.5 feet in West Palm Beach). This pattern is caused by the shape of the coastline. The South Atlantic Bight is an ocean embayment, formed by the coastline between West Palm Beach, FL, and Cape Hatteras, NC. The flow of ocean water is funneled from Florida and North Carolina toward the head of the embayment, piling up on the central coast of Georgia.
Tidal ranges on many stretches of the AICW are substantially lower than the ranges found at the nearest ocean-facing beach. For example, the tidal ranges in the Indian River Lagoon, Mosquito Lagoon, Waccamaw River, Pamlico Sound, Albemarle Sound, and Currituck Sound are all significantly smaller than the tidal ranges on the ocean-facing beaches that are nearest to each of these inland water bodies. This is because the inlets restrict the amount of water that can be exchanged between the ocean and these estuaries, limiting the extent of the tidal fluctuations. Remember that the tides originate in the ocean; a rising tide has to flow through an inlet to get into the estuary and the ICW. In the areas listed above, the inlets are spread far apart and are relatively small. However, even though the amount of water exchanged through these inlets is small compared to the total volume held inside the estuaries, the tidal currents in these inlets can be quite strong. The water is flowing at high velocity through a narrow passage. In some areas, the ICW also passes so far inland that it enters freshwater, where the tidal influence is greatly diminished. The stretches of the ICW with little tidal influence include Virginia, south of the locks; North Carolina, north of Beaufort/Core Creek; South Carolina, between Myrtle Beach and Georgetown; and in the Indian River Lagoon, FL.
Clearly, a 9-foot tide change requires some consideration when navigating, docking, or anchoring. And because the Georgia coast is a maze of coastal rivers, tidal creeks, and inlets, the large tides can result in strong tidal currents, as the flow of water is accelerated when it passes through narrow channels. Even in areas of the Southeast that have relatively small tides, the current can be quite strong at the inlets. In Florida, for example, the tides are fairly small but the inlets are narrow and spaced far apart. There's a lot of water trying to get to and from the estuaries through those few, small inlets. Don’t be surprised by the strength of the tidal currents at any of the inlets, anywhere along the ICW, especially on the outgoing tide.
Paying attention to the tides will not only tell you something about the water depths that you will encounter, but will also help you predict the direction and relative strengths of the currents you will be negotiating. Thankfully, tides are highly predictable. Tides are so predictable that you can consult a tide table (by going NOAA’s Tides & Currents website) to see the time and height of the tides on any day between now and New Years Eve in the year 2101! At this point you may be wondering if tides are so predictable, then why does the water level often differ from the tidal predictions? These observed differences are due to the next three factors that we’ll discuss: wind, rainfall, and relative sea level rise.
Tides for Sapelo Island, GA (near MM 649) for the entire month of December 2099. You can never start planning your cruise too early! The fact that you can get accurate tidal predictions more than 6 decades in the future demonstrates the predictability of tides. Note that the heights for the high tides and low tides vary. Every other low tide is extra low and every other high tide is extra high. The water level at mean lower low water (MLLW) is designated as the 0 mark, and all other water level measurements are expressed relative to this benchmark. The water level at mean lower low water is found by marking the water level during the lowest low tide from each day over a 19-year period and finding their average. Mean higher high water is calculated by measuring the height of the water above the MLLW level during the highest high tide on each day and then averaging those values. The “spring tides,” which are the highest high and lowest low tides of the month, take place on December 11 and 29. For reference, the new moon will be on December 11 and the full moon on December 27.
Wind and Water Level
The direction, speed, and duration of wind events can have a large effect on water levels. Wind-driven changes in water level of two or three feet are not that uncommon in some places. Tide predictions will not account for this change. Whereas the tide’s influence is only significant in certain regions of the ICW, wind can affect the level of the water at any location along the ICW. The wind’s influence is much more obvious where the tides are minimal.
The wind affects the water level on the ICW in two different ways: (1) by blowing directly on the enclosed, inland waters of the ICW and (2) indirectly, by blowing on the ocean waters just offshore of the coast. We’ll start by first describing the direct effects of wind blowing on the intracoastal waters then discuss how wind blowing on the ocean can affect water levels on the ICW.
Wind Blowing Directly on Inland Waters: Enclosed bays and sounds on the ICW have very small tidal fluctuations, including in Virginia, south of the locks; North Carolina, north of Beaufort; South Carolina, between Myrtle Beach and Georgetown; and Florida in the Mosquito and Indian River Lagoons. Water level changes caused by tides in these areas are dwarfed by the effects of wind. Bays, sounds, and lakes are subject to a phenomenon called “seiching,” when the wind pushes water from one side of the embayment to the other. This seiching of inland waters depends not only on the speed, direction, and duration of the wind event, but also on the shape and orientation of the water body itself. The longer the “fetch” (how far the wind blows across the water), the more water that piles up. So, if you have a long, narrow body of water, like the Indian River Lagoon in Florida, the wind will have a much more pronounced effect on water levels when it blows parallel to the lagoon. In other words, water levels will change more on the Indian River Lagoon when the wind blows predominantly from either the northwest or the southeast than from either the southwest or northeast. This is because the lagoon system is long and narrow, running generally northwest-southeast for a distance of approximately 158 miles, but it’s only a few miles wide, southwest to northeast, at its widest point. When the wind blows across a sound or lagoon, it causes the water to tilt: water levels on the windward side of the lagoon drop as the water is pushed toward the leeward side. Pamlico Sound is another good example. The longest axis of Pamlico Sound (i.e., the direction of greatest fetch) is oriented northeast/southwest. A northeast or southwest wind has an unobstructed fetch of about 87 miles between Rodanthe, on Hatteras Island, and Cherry Point, on the Neuse River. When the wind blows consistently from the northeast at moderate speeds, water levels near Cherry Point and Adams Creek (where the ICW heads south from the Neuse River) can increase by a foot or more, whereas water levels along the Pamlico Sound side of the Outer Banks, near Rodanthe, Wanchese, and Nags Head, decrease by a similar amount. Gale force winds can cause changes in water depth of more than 6 feet. Keep in mind that the tidal range inside Pamlico Sound is only a few inches. So the wind has a much greater effect on the water level of western Pamlico Sound than does the tide. (Tidal range on the ocean side of the Outer Banks is 2 to 3 feet but the narrowness of the inlets limits the exchange of water between Pamlico Sound and the Ocean.) So when predicting water levels on the inland sounds and lagoons, you must consider the speed, direction, and duration of the wind, as well as the amount of fetch.
*(Show a diagram of wind-driven seiching of an enclosed sound or lake)
Northern Indian River Lagoon from Scottsmoor Flatwoods Sanctuary to Satellite Beach. The yellow dashed line is the approximate route of the ICW. Note the IRL’s northwest-southeast orientation and narrow width. (The Banana River and southern Mosquito Lagoon are also shown.)
North Carolina’s Albemarle and Pamlico Sound. The approximate route of the ICW is shown as the green dashed line. Note the northeast-southwest orientation of Pamlico Sound and the east-west orientation of Albemarle Sound. Northeasterly winds push water from the Outer Banks toward the Neuse River, near Havelock.
Indirect Effects on the ICW of Wind Blowing on the Ocean Waters: In the ocean off the Atlantic Coast of the U.S., winds from the north or east will push ocean water toward shore, causing it to pile up at the coast and to flood into the estuaries via the inlets. Conversely, winds from the south or west push water away from the coast. When the wind blows on the ocean's surface, it pushes the water. But it doesn't just push the water straight ahead. Because of the Earth's rotation, Wind pushes ocean water in much the same way that a snow plow pushes snow. (Those of you from the Sun Belt will just have to take my word for it.) A snow plow has a blade that is angled slightly. Usually the left side (driver’s side) of the plow blade is further forward than the right side of the plow blade. This angled plow blade means that as the plow moves forward, it pushes the snow forward and off to the right side of the road. In the northern hemisphere, this is exactly how wind moves ocean water: it pushes it forward (with the wind) and to the right (relative to the wind’s direction, because of the Earth's rotation). Therefore, on the East Coast, winds from the north or east take water from offshore and “plow” it toward shore, whereas winds from the south or west take the water from the coast and plow it further out to sea. Water pushed by a northeasterly wind will remain piled up on the coast like a snow bank as long as the pressure from the wind keeps holding it there. Once that wind pressure relaxes, the movement of the water reverses to re-establish equilibrium between the coastal and offshore water levels. For as long as that wind holds the water up against the shoreline like a snowbank, the water level in the adjacent estuary will rise as water flows into the inlet.
This is important for the sections of the ICW that are close to ocean inlets, which is a large proportion of the AICW (including the entire section through Georgia). How strong do the winds have to be and for how long? The stronger and longer the wind blows, the more water it will push. In general, if there are sustained winds greater than 15 to 20 knots blowing consistently for periods greater than 24 hours, there will be noticeable changes in water depths of at least 1 foot at the inlets. If there is any break in that wind, the water will have a chance to return to “normal” levels. To change water levels, the wind has to be blowing over a large area of ocean. A daily coastal seabreeze, for example, is limited to nearshore areas and these convective winds, which are driven by the daily heating of the land, tend to subside at night. For this reason, they don’t alter the water level that much. However, a 3-day nor'easter with winds blowing in the 25 to 35 knot range can really increase water levels along the coast.
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| Wind blowing over the ocean pushes water ahead and to the right (in the northern hemisphere), similar to how a snow plow pushes snow. Here, a northeast wind is shown blowing water toward shore and into an inlet on the East Coast. |
Each pie wedge depicts the direction from which the wind is blowing over the ocean, off of the U.S. East Coast. Blue indicates increasing water depth and red symbolizes decreasing depth at the coast. On the East Coast, in general, steady winds blowing from the 135-degree arc from north to southeast will increase water depths at the coast. Water piles up along the coast, like a snow bank, and it flows through the inlets and into the estuary, raising the water level inside the estuary (and on the ICW). Wind blowing from the 135-degree arc between south and northwest will cause the water level to drop at the coast. Water will stream out of the estuaries and into the ocean via the inlets. The effect is greatest when the wind is blowing from the sectors with the darker colors. This will cause changes in water depth on segments of the ICW that pass near inlets. The exact effects may vary, depending on the shape and orientation of the coastline at your specific location.
What do these wind-driven water flows mean to you as an ICW cruiser? The effects of wind on water levels are less predictable than those of tides. Tides can be accurately anticipated decades in advance. On the other hand, you can look at forecasts for wind-driven coastal flooding, which look out to about 10 days in the future, and have relatively large degrees of uncertainty. Coastal storm-surge flood predictions have even greater forecast uncertainties than do weather forecast models. These storm surge predictions depend on weather models to predict how the water will move around a complex coastline. Therefore, the uncertainties in the weather forecast models are compounded by the uncertainties in the flood prediction models.
Because changes in water level caused by wind are less predictable, the consequences of running aground in a wind-dominated area can be more serious than those of running aground in a tidally dominated area. If you run hard aground in Georgia, you can probably just wait a few hours for the rising tide to float you off (unless you run aground at high tide). But if you get stuck at the mouth of North Carolina’s Alligator River (where it joins Albemarle Sound), you will be more likely to require assistance to get free because the tide will not help float you off.
Wind-driven changes in water level have obvious implications for where your draft allows you to go. For example, If you are trying to pass the notoriously shallow section of the ICW near Isle of Palms, SC, just north of Charleston Harbor (MM 460-461), doing so during a period of prolonged northeasterly winds would give you more depth. Wind-driven changes in water levels also affect bridge clearances. Having a mast higher than 62 feet warrants paying close attention to the effects that wind has on the water level. For example, one place where this is important is at the Wilkerson bridge at the south end of the Alligator-Pungo Canal (MM 126).
The effect of wind on the water level can be significant, or even catastrophic, as in the storm surge associated with a hurricane. Usually it’s a bit less dramatic but still worth considering when planning a passage on the ICW. Wind-driven effects on water level can last from hours to days. Chances are that when there is a strong wind, you will choose to stay at anchor or tied to a dock. Even though you won’t be underway, you will still want to consider the implications of wind-driven changes in depth. Will your keel be bouncing off the bottom or, if at a fixed dock, will you be able to get on and off the boat, or perhaps, will your boat be riding up above the pilings?
Rainfall
Rainfall generally doesn’t make much difference to water levels near the inlets or in large bays/sounds. However, on the sections of the ICW that meander far inland and the water is fresh, rainfall can affect water levels (think Virginia Cut, the Great Dismal Swamp, South Carolina’s Waccamaw River, and even the southern Indian River Lagoon). These areas account for a small portion of the AICW, but a major rainfall event can increase water levels for days to weeks in these localities. One spot where this may be a concern for boats with masts taller than 62 feet is the Socastee Bridge (AKA, the Benjamin Thraikill Bridge) at MM 371, between Myrtle Beach and Georgetown, SC. For the most part, however, unless there has been some extreme rainfall event (e.g., a slow-moving tropical weather system that drops enormous amounts of rain over the entire region), you shouldn’t have to worry too much about the effects of rainfall on water depths or bridge clearances.
Relative Sea Level Rise
Sea level has varied for hundreds of millions of years. But as ICW cruisers living in the present, we don’t really care what happened a million years ago. Within the time scale that matters to us, sea level has been increasing and will continue to do so, albeit at a very slow rate (compared to the changes in water level caused by the previous three factors discussed). But even slow rates of change eventually result in large changes overall. And if you have a mast height of 64 feet, it wouldn’t take much of a decrease in bridge clearance to ruin your day.
As coastal navigators, we are concerned with relative sea level. That is the level of the sea compared to the level of the land. Therefore, relative sea level depends on changes in both the level of the sea’s surface and in the elevation of the land. Unfortunately, along the Southeast and Gulf Coasts of the U.S., the sea level is rising while the land is subsiding. As the land slowly sinks, so too do things that we like, such as boatyards, marinas, and, most important to this discussion, bridges. To give you a sense of the magnitude of this change, on the coast of Georgia, relative sea level has risen roughly 14 inches since 1950. Most of that change is attributed to the land sinking. But the speed of relative sea level rise has been increasing over time because sea level itself is rising, and the rate of this sea level rise has been increasing. In fact, most of the increase in relative sea level that has occurred over the past century took place in just the past 20 years because the rate of the water’s rise has been accelerating. Relative sea level is expected to rise by at least an additional foot by 2050. Currently, relative sea level, which takes into account changes in the elevations of both the sea surface and the land, is rising on the Southeast and Gulf Coasts of the U.S. (North Carolina to Texas) at a rate of almost one-half inch per year (>11 millimeters per year). This means that water levels for just about the entirety of the AICW and GICW are also rising by nearly a half-inch per year, relative to the elevation of the shoreline. And, yes, we are sure about these numbers. (See the articles cited at the end of the chapter and the peer-reviewed papers that they reference.) This rate of relative sea level rise is 2 to 3 times the global average.
All bridges on the AICW, with one exception, are supposed to have 65 feet of clearance at mean higher high water. Since the water has been consistently rising and is expected to continue to rise, the clearance under any bridge slowly decreases throughout its 50- to 80-year life expectancy. As you may have guessed, the older bridges tend to be the ones that are most problematic regarding vertical clearance. In an attempt to keep up with the rising relative sea level, new bridges are being built to a higher benchmark. The new bench mark elevation is still 65' at MHHW, but it recognizes that the actual water level at MHHW is now higher than it was when the previous benchmark was set.
The problem of bridge clearance is greatest where the lunar tides are smallest. In places like Georgia, where the tidal range is large, you routinely have less than 65 feet of clearance at high tide. But high tide doesn’t last very long. For most of the time in Georgia, the water level is many feet lower than it is at high tide. Therefore, bridge clearances in Georgia are 65 feet or greater during the vast majority of the time. However, on those inland areas of Virginia, North Carolina, South Carolina, and Florida, the water can remain relatively constant for long periods. This means that at some of the bridges in these areas (particularly the older ones), there can be long periods during which the bridge clearance is less than 65 feet. And the proportion of time during which the bridge clearance is less than 65 feet is increasing because of relative sea level rise.
Using Ecological Knowledge
As you are traveling along the ICW, it is easy to lose track of the tides since you are never in one location for very long. If you suddenly find yourself wondering if you are near high tide or low tide look for four things: 1) Spartina grass (Spartina alterniflora, which is just about all of the grass you see in the salt marshes along the ICW); 2) the marsh turf from which the tall blades of Spartina grass grow; 3) living oyster reefs; and 4) mud flats. In the region from Swansboro, NC, through St. Augustine, FL (with the exception of the section from Myrtle Beach to Georgetown, SC, which is mostly in freshwater), these common elements of a salt marsh occur in a consistent vertical pattern. Mudflats are in the lowest portion of the intertidal zone and the Spartina grass is in the highest. Marsh turf and dark-colored living oyster reefs occupy the middle zones, with the turf being slightly higher than the tops of the oyster reefs. (The lighter-colored oyster shells that you see piled on top of the marsh turf are dead oysters that have been thrown on top of the marsh by currents and waves. They are not part of this scheme.) In a very general sense, you can think of the mudflats occupying the lowest third of the intertidal zone, living oyster reefs inhabiting the middle third, and the turf and Spartina grass are in the upper third. If you can see broad expanses of bare mud below the bottoms of oyster reefs, then you know that the tide is very low. On the other hand, if you can’t see any mudflats or living oyster reefs, and you can only see the turf and Spartina grass, then you know that you are in the upper third of the tide. If the turf is under water, you are near high tide. During spring tides (full and new moons), only the very tops of the grass plants will be visible at high tide. The direction of the current relative to the nearest inlet will tell you whether the tide is coming in or going out. This type of observation can also be helpful in areas that have very small tidal ranges and changes in the water are driven primarily by wind and/or rainfall. In these areas, you can tell when the water level is lower than normal by looking for dark-colored algae growing on the pilings and bridge clearance (“tide”) boards. This algae needs to be wet most of the time, so if you see that dark algae, you know that the water level is lower than normal. If you can’t see this algae just above the water’s surface, then the water is likely to be at or above normal levels.
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| Empty shells of dead oysters, washed high up onto the marsh by waves. Don’t confuse the sun-bleached oyster shells with the living oyster reefs located in the middle of the intertidal zone. These oyster shells have been washed over top of the Spartina grass. Sapelo Island, GA, at the confluence of the Duplin River and Doboy Sound. |
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| From the thick, dark algae growing on this tide board at the Wilkerson Bridge (MM 126), it’s obvious that having 65 feet of clearance is rare. But at this moment, there are a few more inches of clearance than usual. |
Conclusions
Tides, wind, rainfall, and relative sea level change all affect water levels. These four factors operate independently of each other, over different time scales from hours to millennia. These factors differ in magnitude and predictability. The water level at any point in time is the sum of all these factors. Because they affect water depth, current speed, current direction, and bridge clearances, it is very important for ICW cruisers to understand the basics of how each one operates.
To Learn More
Mariner’s Rule of Twelfths (or “How can I figure out the time at which the tide will be high enough for my boat to get over that sandbar?”)
Tools
Coastal Inundation Dashboard (storm surge flooding prediction model)
National Water Prediction Service (Water Levels)
- Introduction
- Chapter 1: Tips & Tricks
- Chapter 2: Water Levels
- Chapter 3: Avoiding Obstacles
- Chapter 4: Running Aground
- Chapter 5: Bridges
- Chapter 6: Inlets
- Chapter 7: Docking, Anchoring & Mooring
- Chapter 8: Navigation Electronics
- Chapter 9: Weather Basics
- Chapter 10: Typical Weather
- Chapter 11: Environmental Stewardship
- Chapter 12: The Perfect Boat
- Chapter 13: Maintenance
- Chapter 14: Conclusion
- About the Authors
