Impacts of Watershed Management on Land-Margin Ecosystems: The Columbia River Estuary

Abstract

Patterns of land use development that have arisen in the Columbia River Basin over the last century are occurring in large river basins worldwide. The consequent modifications of river flow, physical properties, and discharge of sediment and other constituents appear as cumulative effects in land-margin ecosystems, where estuarine processes intercept, entrap, and transform both riverine and oceanic material. These watershed changes alter both the input to the estuary and the fundamental estuarine processes. Our studies of the Columbia River estuary indicate that these human alterations to watersheds can affect the interaction between river flow and the tides, modifying circulation patterns important to estuarine food webs. Mean river flow has decreased approximately 20% since the 19th century; probably 6 to 8% is due to irrigation withdrawal, the remaining 12 to 14% to climate variability. Regulation of river flow has reduced spring freshet flows to about 50% of the natural level, and has increased fall minimum flows by 10 to 50%. The reduction in spring freshets has lowered modern-day sediment input to the estuary to ~25% of that recorded in the latter part of the 19th century. Navigation structures and filling and diking in the lower river and estuary have decreased the tidal prism by about 15%, increased sediment residence time and shoaling, simplified the channel network, and concentrated flow in the navigation channel. In addition to sediment, temperature, organic matter, nutrients, pollutants, and biotic influxes at the estuarine interface, changes in the river discharge regime have modified estuarine stratification, mixing, and residence time. Such modifications have profound effects on sensitive estuarine processes such as those that occur in the estuarine turbidity maximum (ETM), where trapping of suspended material occurs, organic matter is incorporated in a dynamic microbial loop, and important food web linkages to higher level consumers occur. A landscape perspective on the impacts of watershed alterations needs to be included in our emerging regional and global approaches to ecosystem management if land-margin impacts are to be predicted and mediated.