Severn Run is the nine-mile-long headwater stream of the Severn River, and is a relatively healthy stream compared to many in the region. With ample shade provided by a largely intact lowland forest of red maple, river birch, sycamore and ash, it is a recreational trout stream, stocked with brook trout by the Department of Natural Resources (DNR). Of the Severn Run's 24 square mile watershed extending from Interstate 97 to Odenton, over 1,700 acres of the Severn Run watershed are protected by Maryland as a preserve designated the Severn Run Natural Environment Area.
Yellow perch from the tidal Severn school and swim up Severn Run to spawn in March, and other fish found in the Run include largemouth bass, several shiners, sunfish, white sucker, brown bullhead, gizzard shad, and chain pickerel (a more complete listing is found in the Maryland Biological Stream Survey). Public access is provided at Dicus Mill Road and off Veteran's Highway, but there is little management of this area. Trails along the upper Run are typically overgrown, leaving the floodplain of Severn Run largely in its natural state, with nesting barred owls and great blue herons among other birds.
Jabez Branch , Severn Run's largest tributary, is unique in that it is the only coastal plain stream in Maryland with a naturally reproducing population of brook trout . This fish requires clean, cool, running water and cannot tolerate water temperatures greater than 75 degrees F. Most coastal plain streams become too hot in the summer for brook trout survival and reproduction, and Jabez Branch is unusual for its cool springs and high percentage of forested cover. However, as described elsewhere, the Jabez Branch brook trout have been and continue to be threatened by thermal pollution from road construction and development. The DNR website displays stream survey results for the Jabez Branch.