Relatively few fish are found in most of the Severn's branches, though some may spawn in the lower portions of the larger ones, as near Little Round Bay. Tiny eels ascend the branches to reach ponds, often overcoming difficult obstacles to complete their journey from the ocean. Other familiar residents are the green frog, crayfish and waterstriders. The banks are well-vegetated and the stream bottoms consist of fine silt and sand, with old logs and accumulations of fallen leaves in places.
Increasingly, however, the Severn's streams have been affected by large volumes of stormwater runoff from developed and paved areas lacking adequate stormwater management facilities. In these cases stream banks erode and the natural habitats of invertebrates at the bottom of the food chain are lost.The decreased groundwater recharge in developed areas lessens springwater flow, and streams may consist of pools of oily rust-stained water, or the bottom may be covered by algae. The physical and biological conditions of streams in the Severn watershed are currently being analyzed as part of the Severn River Watershed Management Master Plan in order to prioritize restoration efforts for such degraded streams. The Maryland Department of Natural Resources surveyed a number of streams in the Severn watershed for fish, invertebrates, and physical habitat quality in 1994 and 1997, as described in the Maryland Biological Stream Survey.