Institute of Ecosystem Studies
Stream and Watershed Studies - Projects

Stream Biotic Communities
  • Ken Belt, United States Forest Service
  • Susan Gresens, Towson University
  • William Stack, City of Baltimore, Department of Public Works
The response of benthic invertebrate communities to the initial effects of urbanization has been studied on Oregon Branch (with a rural catchment adjacent to that of Pond Branch) and Deep Run, a tributary of the Jones Falls. A longitudinal study was conducted on these two streams, where they traverse a sharp change from rural to urban land use. Impervious area was used to quantify the degree of urbanization. The study watersheds were divided into a series of subwatersheds with the local percentage of impervious area increasing from 0.7% to 70%. Over a two-year period, benthic samples were collected from sites along this urban gradient. Peak shear stress during storms was also estimated at rural vs. urban sites.

A clear increase in the frequency of runoff events and increased potential for disturbance during storms (i.e., shear stress) were observed in the downstream urban reaches. Percent of fine sand in surficial sediment in riffles significantly decreased downstream. In contrast, the matrix of organic material and fine silt coating cobbles increased at urban sites.

Principal components analysis of biological samples showed that the abundance of EPT and Coleoptera taxa decreased and the abundance of Diptera taxa increased along the urban gradient. Results suggest that the indirect effects of altered hydrology on sediment quality may have the greatest impact in biota. Biotic responses to urbanization were observed at 5 - 8% total watershed imperviousness, well below previously cited threshold levels of 10 - 20% impervious cover.

The Baltimore City Water Quality Management Section has an extensive program to evaluate the water quality and biological health of the three major stream systems in the City of Baltimore; Gwynns Falls, Jones Falls and Herring Run. The City has established a network of water quality sampling stations established under the City of Baltimore's Municipal Stormwater Permit Program. The water quality at the stations is depicted using indicators (high, elevated and low) for metals, nutrients, pathogens and dissolved oxygen. The indicators are based on a methodology developed by the Maryland Department of Natural Resources for their statewide water quality inventory (305b report).

The City has also established a network of benthic macro-invertebrate sampling stations and a biological score indicating the health of the benthic community. The biological score is expressed as a percentage of the average scores of three minimally impacted streams (reference stations) in Baltimore County. The scores are based on the averages of all samples collected at each site, and were computed by a multi-metric scoring method.