Recent surveys indicated four colonies of beaver and a small population of muskrats. River otter use the flowage extensively and can usually be observed along the shoreline in the early morning. River otter have large home ranges so it is difficult to determine how many family groups use the flowage. The Willow Flowage has a high quality fishery with angling being one of the major uses. Walleye are the most common and sought after game fish.
Northern pike are also common along with lesser numbers of large and smallmouth bass and musky.
The flowage also has a quality panfish population consisting of black crappie, bluegill, pumpkinseed and yellow perch. Other species present are black and yellow bullheads, dog fish, white suckers, burbot, silver redhorse, shorthead redhorse, and various minnow species. Greater redhorse are also present and are listed as a threatened species in Wisconsin. No fish stocking is done because excellent natural reproduction is able to maintain the fishery. These animals include various mammals, birds, reptiles, and amphibians.