What are watersheds?

page index 1 | 2 | 3 | 4

Basin versus Nonbasin Drainage

15% to 20% of runoff within a large watershed drains directly into its highest-order stream or water body without first flowing through lower-ordered basins. Areas that drain directly into higher-order water bodies are called nonbasin drainage areas and include lands adjacent to river, bay and ocean shorelines. In Maryland, nonbasin bay drainage areas are subject to regulation under Maryland's Critical Areas Act. This act restricts development within 1000 feet of waters under tidal influence, and mandates 100-foot wide vegetated buffers between tidal waters and adjacent land-uses to help control surface runoff and thus control water pollution.

Nonbasin and basin drainage areas. Source: Marsh 1998, 170.

Stream Functions, Watersheds, and Urbanization

Stream systems have many functions. They provide habitats for aquatic organisms, are important components of terrestrial ecosystems, and function to convey runoff and sediment loads out of their watersheds. A stream's load consists of three kinds of materials: dissolved materials carried in solution, fine particles held in suspension, and heavier or coarser materials pushed or bounced along the channel bottom. Over time, a stream becomes graded. That is, a balance or equilibrium is reached among channel slope (gradient), channel characteristics, available discharge, and load. Stream banks and channels are relatively stable under graded conditions. This balance is upset, however, by changes to the land cover and surface characteristics of the watershed. The urbanization of watersheds increases the imperviousness of land surfaces, alters the density of channels, and diverts much of the surface drainage to underground storm sewers. This, in turn, dramatically changes the volume of water and the amount and type of material that streams in urbanized watersheds convey. Urbanization also alters the physical configuration and stability of stream channels, reducing their value as wildlife habitats. The sections that follow will address the environmental impacts of increased watershed imperviousness as a result of urban and suburban development.

page index 1 | 2 | 3 | 4

© CGIS at Towson University