Muskrats (Ondatra zibethicus)

Size: 18–26″ long, including the 10–12″ long tail
Weight 1 1/2–4 lbs.; most average about 2 1/2 pounds.
Looks like a large rat with a long, narrow tail (the tail is flattened vertically).

Signs of their presence:
Tracks: Hind print about twice as long as front, may overlap front track. May see foot drag and tail drag.

Scat: Oval pellets, often seen in clusters on rocks, logs, or any object that sticks out above the water line. Usually, the 1/2″ long pellets are stuck together, but may flatten with age.

Dens (also called “bank burrows”). Muskrats may burrow into the banks of streams or ponds. The entrance, usually 5–6″ in diameter, is often found about 6″ below the water’s surface; this tunnel, which connects to the den, could be up to 45 feet long. Muskrat dens are found above the high-water line (this may still be 4–5 ft. below ground level). The den is usually only a bit bigger than the tunnel that leads to it, often 6–8″ wide. A small air shaft at the top connects it to the surface; it’s often “screened” with loose twigs or plants. Dens generally have several underwater entrances. In fast-moving water, muskrats are more likely to burrow into the bank than to construct a lodge.

Muskrats may build a den in the outer wall of a beaver lodge, even when the beavers are using their lodge.

Lodges. If the water’s shallow and plants are abundant, muskrats may build a lodge that looks a bit like a smaller version of a beaver lodge. Muskrat lodges are usually a dome-shaped hut of weeds, sticks, and leaves piled on a platform of mud and rotted debris. They’re often slightly lopsided, 3–6 ft. wide, and about 1 1/2–4 ft. above the high-water line. The walls of the lodge are often a foot thick. Its inside is hollowed into several chambers. There are several underwater entrances, called “plunge holes.” Muskrats will add to the lodge as long as they’re using it. (Same is true for feeding platforms.)

“Runs.” As the muskrats enter and leave their dens, their hind feet scour out a path in the muck in the bottom of the pond. You may be able to see the run in clear water, or feel the smoothed trail with your hands or feet. Muskrats will also travel over land, especially in the fall and right after ice-out. Trails, both in the water and on land, are kept open by frequent use and pruning, and are noticeable. You may see tracks and scat in their trails.

Feeding stations. They may look like a small lodge. Muskrats will tow food out to this platform. They’ll also push mud and aquatic plants up through a hole in the ice (called a “push-up”). When the mud freezes, it keeps the hole open and creates a shelter for the muskrats, protecting them from both predators and the cold. They’ll rest in the push-up instead of returning to the lodge, which may be further away. May see floating blades of cattails, sedges, or other plants near their feeding platforms, or piles of clamshells on the platform.

Scent posts. Male muskrats will secrete an oily, pungent liquid on a scent post, which is often a twist of grass at the water’s edge, to mark their territories. The musky odor of muskrats (both genders) is especially noticeable during the breeding season.
Common nuisance situations:

Time of year: Spring.
Most muskrat damage is caused by their burrowing.

They burrow into earthen dams, dikes, levees, and railway embankments, weakening their structures. They also burrow into the banks of ponds, canals, and irrigation and drainage ditches. Their tunnels may drain a small farm pond. They may damage floating docks, marinas, and boathouses.

Muskrats will cross yards. Some people are frightened of them, or mistake them for Norway rats.

Occasionally eat field crops. May cause substantial financial losses in states with major rice and aquaculture operations, because they eat rice, cut it down to use as building material for their lodges, and damage the field by burrowing through levees.

Damage aquaculture sites by burrowing into levees or pond banks.

Damage ornamental aquatic gardens by eating water lilies or cattails or other plants.

When their populations grow too high, they may “eat out” all of the aquatic plants in the area, reducing the quality of the habitat for other species, such as waterfowl.

Disease risks: tularemia, hemorrhagic septicemia, leptospirosis, salmonellosis, ringworm, pseudotuberculosis. They are hosts for many ticks, mites, fleas, and various worms.

For more information:
http://nwco.net/09-appendixcbeyondtheroutine/9-08-Muskrats.asp