Projecting the biological condition of streams under alternative scenarios of human land use

We present regression models for estimating the status of fish and aquatic invertebrate communities in all second to fourth-order streams (1: 100 000 scale; total stream length = 6476 km) throughout the Willamette River Basin, Oregon (USA). The models project fish and invertebrate status as a function of physiographic, land-use/land-cover, and stream flow variables, with the latter two sets of variables subject to change under historical and alternative future scenarios of human development. Models are developed using sample data collected between 1993 and 1997 from 149 wadeable streams in the basin. Model uncertainties are propagated through model projections and into aggregated estimates of regional status. The projections show no significant change in basin-wide status in year 2050, relative to Circa 1990, for scenarios either of increased human development or continuation of current development trends, because landscape change under these scenarios is dominated by conversion of agricultural land to rural residential and urban uses, and because these changes affect only a small percentage of the basin. However, under a scenario of increased conservation, regional medians of biotic status indicators are projected to improve by 9-24% by year 2050. None of the changes projected between Circa 1990 and year 2050 is as large in magnitude as the decline in status projected to have occurred between the time of pre-European settlement and Circa 1990.