chapter 3 fundamentals of fluvial geomorphology and stream ...

3.7.4.2 Upstream FactorsThe stability of a fluvial system can also be significantlyaffected by changes to upstream reaches that alter thedownstream discharge or sediment supply. The flow regime andsediment load together, constitute the two main driving variablesresponsible for forming and maintaining the stream and it is nosurprise that the stability of an alluvial river may well be disturbedby changes in one or both of these factors. Upstream factors areoften affected by engineering and river-management projects andthe case of river regulation by a dam or diversion structure is acommon cause of downstream channel adjustment that serves toillustrate the types and complexity of system response that mayresult from such changes.Channel response downstream of a dam or diversionstructure depends on the way the works are constructed andoperated. When the structure is built, sediment supplydownstream may be elevated due to disruption of the channel andfloodplain during construction. This may increase supply overtransport capacity, inducing an initial adjustment throughaggradation. However, this response will be absent if appropriatesediment control measures are applied on site. Once the works arecomplete, process-response downstream will depend on thebalance of changes to the water and sediment regimes. Followingclosure of the dam or diversion, sediment is trapped in the poolupstream of the structure. Sediment-free water released from thestructure then scours the bed downstream, generating degradationin the first few kilometers below the dam. Initially, the flush ofsediment produced drives aggradation further downstream, but asthe stream slope immediately downstream of the dam flattens,sediment output decreases and the leading edge of the zone ofdegradation migrates downstream to re-erode recently depositedsediment and sends it further downstream, as an aggradationalwave. This conceptual model of river response to closure of a damhas been observed in many rivers, and yet this pattern ofadjustment is by no means universal. To explain why, it isnecessary to consider the other morphological responses that maydominate adjustment of the fluvial system. For example, if the beddownstream of the dam includes a widely graded, coarse-grainedfraction, bed armoring may limit degradation and stabilize the bedat a steeper slope than that prior to dam construction. The sameeffect may result from the presence of a geological control, whilewidening with limited reduction in bed level may be triggered if thestream banks downstream of the dam are close to the criticalheight for mass instability (Thorne and Osman 1988). If regulationby the dam significantly reduces the magnitude or frequency ofsediment transporting flows, degradation may be limited ornegligible, and if a reduction in the competence of the main streamis coupled with the input of a substantial sediment load fromunregulated tributaries, aggradation may occur downstream of theFundamentals of Fluvial Geomorphology and Stream Processes 53