Home » Ecology: What makes Casco Bay an estuary?

Ecology: What makes Casco Bay an estuary?

Estuaries are defined as “where the river meets the sea”

Estuaries form where river meets the sea and fresh water mixes with salt. Teeming with life, these places of salt marshes, eelgrass beds, mudflats, and tidal waters serve as nursery areas for oceangoing fish, migratory stopovers for shorebirds, and homes for an amazing diversity of shellfish, fish, mammals, horseshoe crabs, crabs, plankton, and many others sea creatures.

Casco Bay is saltier than most estuaries because the rivers that flow into Casco Bay are small and do not contribute much fresh water to the system.

Salinity variations in Casco Bay are less a result of changing tides, as in many estuaries, than the distance from freshwater inputs (rivers and rain runoff)


Find the rivers of Casco Bay

Look at a map of the towns that border Casco Bay, from Cape Elizabeth to Phippsburg. [link] Find the rivers that flow into Casco Bay (Fore River, Presumpscot River, Royal River, and Harraseeket River).

Lay a string along the twists and turns of each river from its source to the sea, and then straighten it out on a ruler to find the length. Which is longest?

Which one is closest to where you live?

Look at a chart of Casco Bay.

Here’s a trick question: Which river delivers the most freshwater to Casco Bay?

The river that delivers the most freshwater to Casco Bay isn’t even in Casco Bay; it’s the Kennebec River, which flows into the ocean between Phippsburg and Georgetown, just beyond Casco Bay. It carries water from several rivers, including the Kennebec, Cathance, and Androscoggin rivers. All are considerably longer than any rivers that flow into Casco Bay. This freshwater input curls around and into eastern Casco Bay.


Fresh to Salty

Find 3 sites on the chart: a river mouth, a nearshore site, and offshore:

  • Mouth of the Presumpscot River, Falmouth
  • Fort Gorges, which is just off the Presumpscot and Fore rivers
  • Ram Island Ledge at the entrance Portland Harbor (not far from Portland head Light)


Which one do you think is the saltiest? Which one is the least saltiest?


A river has zero salinity; Casco Bay’s average salinity ranges from 25 to 32 parts per thousand (ppt), and open ocean water has salinity of about 32-35 ppt. Thus, the average salinity of seawater is about 0.3%.


Another way to figure out which site is saltiest is to look out the actual data collected by Friends of Casco Bay.


How did we collect this information? Volunteers collected water samples along the shoreline between April and October; staff scientists sampled by boat year-round.


Average Surface Salinities at 3 Sites in Casco Bay, 2001-2005

Measured in parts per thousand (ppt)

Compare the average monthly salinity data for those three sites in Casco Bay.


Month                  Presumpscot River                          Ft. Gorges           Ram Island Ledge


January                                                                                                29.4 ppt                                32.0 ppt

February                                                                              31.0                                        32.1

March                                                                                   27.8                                        30.7

April                       1.3 ppt                                                  28.7                                        30.0

May                       2.6                                                          30.0                                        30.6

June                      1.3                                                          28.0                                        30.3

July                        2.4                                                          29.9                                        30.6

August                  2.3                                                          30.1                                        31.2

September         4.2                                                          30.5                                        31.4

October               3.1                                                          29.5                                        31.7

November                                                                          29.5                                        31.3

December                                                                           28.1                                        32.9


What’s left behind?

What factors affect the salinity in Casco Bay? (rivers, tides, rain, melting snow, evaporation, ocean currents).

Collect jars of water from a river, from just where the river meets the bay, and from the ocean. Put each sample into a separate aluminum pan and allow the water to evaporate over the next day or two. What is the residue left at the bottom of pan?  How do they differ?

After all the water has evaporated, mix up the pie plates and challenge others to figure out which is from the river (hint: no salt, maybe a little sand), the mouth of the estuary (some salt), and open ocean (most salt).

Caution: Don’t let anyone try to figure out which is saltiest by tasting the granules! Why not?


Books that trace the river to the sea

  • The Secret Bay by Kimberly Ridley and Rebekah Raye, Tilbury House, 2015
    The Secret Bay uses a variety of formats to explain the life of estuaries:  poetry, natural-history sidebars, and field guide
  • A Day in the Salt Marsh. by Kevin Kurtz, Sylvan Dell Publishing, 2007
    Hourly changes brought on by the tides


What is the value of estuaries?

Brainstorm a list of ways that an estuary is valuable for wildlife (including marine life, birds, and terrestrial animals). If you need ideas, check out the resources below: