The influence of catchment land use on stream integrity across multiple spatial scales

Abstract

1. Despite wide recognition of the need for catchment‐scale management to ensure the integrity of river ecosystems, the science and policy basis for joint management of land and water remains poorly understood. An interdisciplinary case study of a river basin in south‐eastern Michigan is presented.2. The River Raisin drains an area of 2776 km2, of which some 70% is agricultural land. The upper basin consists of till and outwash, and both topography and land use/cover are diverse. The lower basin consists of fine textured lake deposits, is of low relief, and land use is primarily agricultural.3. The River Raisin basin historically was a region of oak‐savannah and wetlands. It was deforested, drained and converted to farmland during the mid‐nineteenth century. Human population reached a plateau at about 1880, and then underwent a second period of growth after 1950, mainly in small urban areas. More recently, the amount of agricultural land has declined and forested land has increased, in accord with a general decline in farming activity.4. It could be suggested that the influence of land use on stream integrity is scale‐dependent. Instream habitat structure and organic matter inputs are determined primarily by local conditions such as vegetative cover at a site, whereas nutrient supply, sediment delivery, hydrology and channel characteristics are influenced by regional conditions, including landscape features and land use/cover at some distance upstream and lateral to stream sites.5. Sediment concentrations measured during low flows were higher in areas of greater agriculture. In a comparison of two subcatchments, sediment yields were up to ten times greater in the more agricultural location, in response to similar storm events. A distributed parameter model linked to a geographical information system predicted that an increase in forested land cover would result in dramatic declines in runoff and sediment and nutrient yields.6. Habitat quality and biotic integrity varied widely among individual stream sites in accord with patterns in land use/cover. Extent of agricultural land at the subcatchment scale was the best single predictor of local stream conditions. Local riparian vegetation was uncorrelated with overall land use and was a weak secondary predictor of habitat quality and biotic integrity.7. Investigation of the regulatory agencies involved in land and water management in the basin revealed a complex web of overlapping political jurisdictions. Most land‐use decision‐making occurs at the local level of township, city or village. Unfortunately, local decision‐making bodies typically lack the information and jurisdictional authority to influence up‐ and downstream events.