GEOMORPHIC AND ENVIRONMENTAL-EFFECTS OF INSTREAM GRAVEL MINING

Instream gravel mining involves the mechanical removal of gravel and s and directly from the active channel of rivers and streams. Active cha nnel deposits are desirable as construction aggregate because they are typically durable (weak materials having been eliminated in river tra nsport), well-sorted, and frequently located near markets or on transp ortation routes. Instream gravel mining commonly causes incision of th e channel bed, which can propagate upstream and downstream for kilomet ers. As a result, bridges and other structures may be undermined, spaw ning gravels lost and alluvial water tables lowered. In analyzing the effects of instream gravel mining, a sediment budget analysis sheds li ght on the relative magnitude of gravel supply, transport and extracti on. Computer models of sediment transport are simplifications of compl ex natural processes; they can be useful components of a sediment budg et analysis but should not be relied upon alone. A historical analysis of channel change and sediment supply is needed to understand the und erlying processes responsible for present conditions. While instream g ravel mining can be a useful tool in flood control and river stabiliza tion in aggrading rivers, most rivers in the developed world (certainl y the vast majority below reservoirs) are not aggrading and are more p rone to incision-related effects of instream gravel mining.