Sage the Owl
Hoo! I'm Sage. I like to follow questions all the way to the answer.
Today's question: after a rainstorm, where does all that water go?
Where does a raindrop go?
Imagine one raindrop landing near Stono River County Park on Johns Island. It could soak into soil, run across a path, or flow to the tidal marsh. A good scientist follows the drop and measures what happens.
Watershed
A watershed is a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to places like bays and the ocean. (NOAA)
Surfaces rain cannot soak into
EPA explains that buildings, pavement, and compacted ground do not let rain soak into the ground. Water that cannot soak in becomes runoff, and it moves across the surface.
Surfaces rain can soak into
USGS says a large part of the rain in forested watersheds is absorbed into the soil. This is called infiltration. The water is stored and is slowly released to streams.
South Carolina's salt marsh
South Carolina has about a half-million acres of salt marsh. That is more marsh than any other Atlantic coast state. (SC DNR)
Stono River County Park
The park is about 85.5 acres with 1.5 miles of trail. A boardwalk crosses the tidal marsh to a small island. (Charleston County Parks)
Tap a card, then tap the bin it belongs in: Can be measured or Only an opinion.
Tap a card, then tap the bin it belongs in: Water mostly soaks in or Water mostly runs off.
Tap a card, then tap the bin it belongs in: Real fact or Imaginary example.
Tap a card, then tap the bin it belongs in: A source says or My prediction.
Rivet the Snake
Ssssup! Rivet here. I slither along creek banks, so I know where the water goes.
Rain makes a path. Let's trace it from the yard to the marsh.
From the yard to the river to the marsh
Rain that lands on a watershed has a path to follow. It can soak in, or run off. Either way, much of it ends up in the river and the marsh. Read the cards, then show what you know.
Two highs, two lows
Most coasts have two high tides and two low tides every lunar day. A lunar day is about 24 hours and 50 minutes. High tides come about 12 hours and 25 minutes apart. (NOAA)
An estuary
An estuary is usually found where a river meets the sea. The water is a mix of fresh water from land and salty water from the sea. (NOAA)
Spartina
Smooth cordgrass, called spartina, can live with regular flooding by salt water. It dominates the low marsh, and its stalks are tough and well anchored by roots. (SC DNR)
Two marsh zones
The low marsh floods every day. The high marsh floods only during twice-monthly "spring" tides and during storms. (SC DNR)
Pluff mud
Pluff mud forms in salt marshes where tides bring in fine sediments, decaying plants like smooth cordgrass, and tiny living things. Bacteria breaking down plants cause its rotten egg smell. (S.C. Sea Grant)
A marsh as a sponge
Salt marshes filter sediments and pollutants from water. During a storm surge, the marsh acts like a giant sponge that soaks up and holds a large amount of water. (SC DNR)
Tap the ONE place in the picture where rain soaks into the ground most easily.
Tap the ONE surface in the picture where rain cannot soak in and runs off fastest.
Tap the tall smooth cordgrass (spartina) that grows in the low marsh and floods every day.
Tap the wading bird. Herons and egrets feed on fish, shrimp, and fiddler crabs in the marsh.
Clover the Horse
Neigh! I'm Clover. I like open fields, and I know rain on a field soaks in fast.
Let's use real rain numbers to see how much water we are talking about.
Measure the rain, the tide, and the marsh
Scientists turn rain and tides into numbers. We use rounding, adding, subtracting, multiplying, dividing, time, fractions, area, and perimeter to read the data. Keep scratch paper nearby.
Charleston County rain, from the SC State Climate Office:
An average year: about 45 inches
The wettest year (1994): about 81 inches
The driest year (2000): about 28 inches
The most rain in one day (Oct. 3, 2015): about 11 1/2 inches
Real numbers come from the cards. Every "Imaginary example" is made up for practice.
Average year
Charleston County gets an average of 45.30 inches of rain a year. The most in one day was 11.50 inches on October 3, 2015 at Charleston Air Force Base. (SC State Climate Office)
Wettest and driest
The wettest year on record was 1994 with 80.78 inches (Sullivan's Island). The driest was 2000 with 27.63 inches (downtown Charleston). (SC State Climate Office)
A different place, a different number
The National Weather Service Charleston report lists a normal of 52.51 inches a year. In 2025, 42.45 inches fell, which was 10.06 inches below normal.
Roof runoff
A 1,000-square-foot roof makes about 600 gallons of runoff in one inch of rain. (S.C. Sea Grant)
Tide heights in Charleston
King tide alert level is 6.6 feet above a standard low-water mark. Minor flooding starts at 7.0 feet, moderate at 7.5, and major at 8.0 feet. (MyCoast SC)
Oyster filter
One adult oyster can filter about 50 gallons of water in a day. (SC DNR)
People on Johns Island (city part)
The City of Charleston counted 5,266 people in the Johns Island part of the city in the 2010 Census and 11,884 people in the 2020 Census. This counts the part of the island inside Charleston city limits, not the whole island. It is a number a scientist can use to study how land use and water might change.
Pinch the Fiddler Crab (cartoon)
Click-click! I'm Pinch, a cartoon fiddler crab. Real ones shred dead marsh grass into tiny bits.
My neighbors need food, shelter, and the right water. Want to meet them?
Who lives here, and what do they need?
Animals need food, water, and shelter. When the land around them changes, the water can change too. Use the cards as evidence. Look, listen, and never touch wildlife.
A nursery
The young of many animals, like blue crabs, white shrimp, and red drum, use the salt marsh as a nursery. (SC DNR)
Three-quarters
Three-quarters of the animals harvested as seafood in South Carolina spend all or part of their lives in estuary waters around salt marshes. (SC DNR)
Diamondback terrapin
Terrapins live their whole lives in the estuary. In South Carolina they like to eat periwinkle snails and small fiddler crabs. Nesting season runs from early May through early July, and eggs take 60 to 100 days to hatch. (S.C. Sea Grant)
Wading birds
Herons, egrets, and ibises feed on fishes, shrimp, and fiddler crabs in the marsh. Clapper rails hunt fiddler crabs among the spartina stalks. (SC DNR)
Oysters
A single adult oyster can filter 50 gallons of water a day. Oysters grow along the edges of the salt marsh. (SC DNR)
Native plants
Clemson Extension says native plants and animals evolved together over thousands of years. Caterpillars that eat native plants are food for many other animals.
Oak and ginkgo
A native oak tree supports over 550 kinds of caterpillars. A non-native ginkgo tree supports only 5 kinds. (S.C. Sea Grant)
Land and water data
USGS says that when a watershed is urbanized, much more water reaches a stream more quickly, which makes flooding more likely and more severe. It also reports that from 1982 to 1997 U.S. population grew 17 percent while urbanized land grew 47 percent.
Marsh history
SC DNR reports that before the marsh food web was understood, over half of the original salt marshes in the United States were filled in to create land for homes, industry, and farms. Federal and state laws now protect salt marshes.
Ways people reduce runoff
A rain garden is a dip in the landscape that collects rain from a roof, driveway, or street and lets it soak into the ground. It can also give food and shelter to butterflies and songbirds. A rain barrel holds roof water for later use on gardens. (EPA, Clemson Extension)
Comet the Unicorn (Imaginary example)
Hello! I'm Comet. I'm an imaginary unicorn, so my hooves cannot hold a measuring cup.
You will have to do the pouring. I will cheer from the clouds.
Soak in or run off? Test it.
You can model rain with a cup of water and two trays. Do the experiment at home, then answer the questions. (No marsh needed for this one!)
Predict
Tray A is bare and models a hard surface like pavement. Tray B is covered with soil. Predict which tray will have more water run off, and write it down.
Set up a fair test
Tilt both trays the same way, using the same book. Put a catch bowl at the low end of each tray. Use the same cup, the same amount of water (1 cup), and pour slowly while counting to 10 each time.
Pour and measure
Pour 1 cup of water on the high end of Tray A. Measure the water in the catch bowl using the 1/4-cup marks. Write it down. Do the same for Tray B.
Repeat
Do each tray 3 times. Make a table with columns for Trial 1, Trial 2, and Trial 3 (cups of runoff) and a row for each tray. Scientists repeat tests to check their results.
Compare
Subtract each runoff amount from 1 cup to find how much soaked in or stayed behind. Did the evidence match your prediction? A model is not the real world, so say what is different about a real parking lot or marsh.
Tap two items to swap them until they are in the order you would really do them, then check your order.
Tap two items to swap them until they are in the order you would really do them, then check your order.
Tap a card, then tap the bin it belongs in: Measures amount of water or Measures time.
Tap a card, then tap the bin it belongs in: Change on purpose or Keep the same.
Sage the Owl
Hoo! Sage here, one more time. A boardwalk over a marsh is a great place to test your thinking.
Pack your field guide and let's plan what to look for.
Build a field guide. Test a prediction.
A scientist brings questions outside instead of deciding the answers ahead of time. Draw a model, predict what you will observe, and bring your field guide with you.
Tap exactly THREE things you want to investigate at the real marsh park.
Measure, observe, and stay safe. Respect the wildlife.
Rivet the Snake
Ssssuper! You followed the water all the way to the marsh.
Now go test your thinking at the real boardwalk, then come tell me what you found.
Certified Marsh Scout
You've practiced making predictions, analyzing evidence, calculating with real data, and comparing habitats. Now test your thinking in the field.