Patuxent River marsh in winter.

Snow and ice along the Patuxent River.

Snow and ice along the Patuxent River.

A winter Patuxent River sunset.

Installing Biologs to abate erosion problems along Spa Creek.

Installing Biologs to abate erosion problems along Spa Creek.

Installing Biologs to abate erosion problems along Spa Creek.

Efforts to preserve shoreline along the Severn River include installation of segmented jetties to diffuse wave energy.

Efforts to preserve shoreline along the Severn River include installation of segmented jetties to diffuse wave energy.

Efforts to preserve shoreline along the Severn River include installation of segmented jetties to diffuse wave energy.

Efforts to preserve shoreline along the Severn River include replanting of marsh grass.

Searching for submerged aquatic vegetation (SAV's) in the Tred Avon River. Part of an annual Bay-wide ground-truthing effort.

Searching for submerged aquatic vegetation (SAV's) in the Tred Avon River. Part of an annual Bay-wide ground-truthing effort.

Searching for submerged aquatic vegetation (SAV's) in the Tred Avon River. Part of an annual Bay-wide ground-truthing effort.

Eroded cliffs along the Tred Avon River reveal buried piles of oyster shells.

Eroded cliffs along the Tred Avon River reveal buried piles of oyster shells.

Eroded cliffs along the Tred Avon River reveal buried piles of oyster shells.

Eroded cliffs along the Tred Avon River reveal buried piles of oyster shells.

Wintertime sailing on the Patuxent River.

Wintertime sailing on the Patuxent River.

National Weather Service employee Sarah Roy checking out the cat-tails in a Patuxent River marsh.

National Weather Service employee Sarah Roy up a creek, but with a paddle. Kayaking in a Patuxent River marsh.

A summer evening's fishing effort along the Patuxent River.

National Oceanographic Data Center employees at sea - kayaking along the Patuxent River.

National Oceanographic Data Center employees up a creek - kayaking along the Patuxent River.

National Oceanographic Data Center employees up a creek - kayaking along the Patuxent River.

A white marsh hibiscus flower in bloom.

A pink marsh hibiscus flower.

A tundra swan family cruising along the Patuxent River.

Ospreys nesting along a Patuxent River duck blind.

Profile view of an osprey perched in a dead tree.

Evening silhouette of an osprey perched in a dead tree.

A Great Egret.

American Oystercatchers are the black and white birds with the reddish-orange beaks.

A Patuxent River Great Blue Heron nesting site, or "heronry."

A Patuxent River Great Blue Heron nesting site, or "heronry."

Footprints in the sand.

Footprints in the sand. The large footprint at the top was apparently made by Homo sapiens.

A female diamondback terrapin, probably looking for a good place to deposit her eggs.

A well-camouflaged Leopard Frog sitting near the edge of a salt water creek.

Horseshoe crabs coming into the shallows at Flag Ponds Nature Park, a unit of Montgomery County, Maryland, public schools.

A Class B pair of horseshoe crabs spawning in the shallows at Flag Ponds Nature Park, a unit of Montgomery County, Maryland, public schools.

Horseshoe crabs coming into the shallows at Flag Ponds Nature Park, a unit of Montgomery County, Maryland, public schools.

The lighthouse at Key West.

The lighthouse at Key West.

The distance and direction to Havana, Cuba, from the southernmost pier on Key West.

The lighthouse at Fort Jefferson, Dry Tortugas National Park.

The mandatory pose at the entrance to Fort Jefferson.

The lighthouse at Fort Jefferson, Dry Tortugas National Park, as seen from the interior of the fort.

Sign over the cell that housed Dr. Samuel Mudd, controversial doctor who treated John Wilkes Booth following Abraham Lincoln's assassination. Mudd was convicted of helping Booth and served three years of his sentence at Fort Jefferson. He helped treat many Union soldiers during a yellow fever epidemic and was subsequently paroled.