Chapter 7 · Try it yourself

Rock, Sand and Rising Water

Twice a day, about every 12 hours and 25 minutes, the sea climbs a Maine shore and falls back again. Everything living on the rock has to survive both worlds: salt water, then sun, wind and hungry gulls. Farther south, sandy barrier islands face a slower tide: a rising sea.

Part 1: A Maine rocky shore

Splash zone
High intertidal
Mid intertidal
Low intertidal
Selected organism

Work the tide

Place

Water level, from low tide to high tide

1.4 m

Height above average low tide.

Halfway, rising

Underwater
–

Out in the air
–

Who lives where?

Part 2: A barrier island and a rising sea

From Long Island to Delaware and beyond, long, low islands of sand guard the mainland. Left alone, storms wash sand from the ocean beach over the island into the marsh behind it, and the whole island slowly rolls toward the mainland. Build a seawall to hold it still, and the sea keeps rising anyway.

Side view, vertical scale exaggerated about 20 times. Distances and heights are illustrative.

How people manage the island

Sea-level rise

0 m

NOAA expects about 0.25–0.30 m of rise along US coasts between 2020 and 2050.

Natural island

Ocean beach width
60 m

Island moved landward
0 m

Storms so far
0

Simplified models. Tides: a smooth, regular tide between average low and average high water; real Maine tides are larger at new and full moon and smaller at the quarter moons. Mean tidal ranges from NOAA tide stations (1983–2001 datum epoch): Portland 2.78 m, Eastport 5.59 m. Zone boundaries, organism heights and “time out of water” are illustrative and computed from that smooth tide; real zones shift with wave exposure and shade. Organism facts: MarLIN (Marine Life Information Network) species accounts for knotted wrack and dog whelk. Barrier island: shoreline retreat of 90 m per meter of sea-level rise and 25 m per overwash storm, the 60 m beach and the marsh response are illustrative, not a forecast; real retreat depends on slope, sand supply and storms. Sea level: NOAA 2022 Sea Level Rise Technical Report (0.25–0.30 m average US rise, 2020–2050); long-term tide-gauge trends of about 2.0 mm a year at Portland, Maine (1912–2025) and 4.2 mm a year at Atlantic City, New Jersey (1911–2025), NOAA Tides and Currents. Assateague: Maryland Geological Survey.