ROBERT B. JACOBSON, U.S. Geological Survey, MS 100, 1400 Independence Road, Rolla, Missouri 65401, USA

In-channel or near-channel mining of sand or gravel inevitably alters the sediment budget of an alluvial system, and may substantially alter channel hydraulics. These alterations can have variable effects on aquatic habitat depending on magnitude and frequency of mining disturbance, mining methods, particle-size characteristics of the sediment load, characteristics of riparian vegetation, and magnitude and frequency of hydrologic events following mining disturbance. In addition, temporal and spatial responses of alluvial systems can vary because of thresholds, feedbacks, lags, upstream or downstream transmission disturbances, and physiographic controls. Minimization of the detrimental effects of aggregate mining requires detailed predictive understanding of the complex response and recovery of a channel to mining disturbance. Decisions about how much, how frequently, and where to mine also require definition of a reference state -- a minimally acceptable physical and biological condition of the channel. However, understanding of alluvial systems is rarely sufficient to predict responses quantitatively and confidently, and reference states are difficult to define. Still, a general understanding of fluvial processes can still provide some guidelines to minimize detrimental effects of mining. Improved understanding sufficient to evaluate physical, biological, and economic tradeoffs, however, will require well-documented case studies and field experiments.