The geomorphic response of gravel-bed rivers to dams: perspectives and prospects

Dams and reservoirs represent the single most profoundhuman alteration of the fluvial system. In almost allcases, dams interrupt and modify the downstream fluxof sediment through watersheds; they typically alsoaffect the flow regime. Because they directly influencethetwooverarchingcontrolsofchannelform–sedimentand water – dams have the potential to alter the entirehierarchy of channel variables (sensu Schumm andLichty, 1965). The geomorphic literature is replete with case studiesof river response to dam regulation, with review papersand monographs providing useful reference (e.g., Petts1980,1984;WilliamsandWolman,1984;Brandt,2000;Grantet al., 2003;Petts and Gurnell,2005; Schmidt andWilcock,2008.)Beyondcasestudies,however,thereisagrowingneedtodeveloppredictivetoolsandmodelsthatcan be used to forecast likely channel response to damconstructionandoperation.Motivatingtheneedforsuchtoolsis the recognition that damsand their flow regimesare likely to be a focal point for energy, ecology, andeconomy in the future. There is increasing interest inchanging or modifying flow regimes at existing dams tomeetvariousobjectives(morenaturalornormativeflowregimes, controlled floods, or ecological or geomorphicrestoration) (e.g., Poff et al., 2007). Many rivers arelikely to experience altered flow regimes due to climatechanges, and modifying dam operations may be oneapproach to mitigate or accommodate such alterations(Payne et al., 2004). At the same time, hydropower iswidelyseenasarelativelycleanandlow-emissionsourceof energy, although there is some debate on this point(Fearnside, 1997). Both economic development andenvironmental pressures are already resulting in majordam construction in the developing world, and thesepressures are likely to grow in the future (Oud, 2002). These converging trends underscore the need forimprovedpredictivecapacitytohelpguidemanagementdecisionswithrespecttodamconstructionandoperationaround the world.Developing such predictive models represents a sig-nificantchallengeforfluvialgeomorphology.Predictingtheeffectsofdamsonriverscanbeviewedasanacidtestforthedisciplineasawhole. Adamrepresentsadiscrete,geographicallylocalized,andmeasurableperturbationofthefluvialsystem.Theeffectofadamonthedistributionand magnitude of flows is known or can be reasonablyspecified, and the trap efficiency of the reservoir, andhence magnitude of change to sediment flux, is equallypredictable. The ability of geomorphologists to rigor-ously forecast changes to the downstream river’s plan-form, hydraulic geometry, grain-size distribution, andslope from changes in the controlling variables ofdischarge and sediment supply represents a real-worldapplicationandtestofthemostfundamentalprinciplesofthe science. Because the boundary conditions are gener-ally known, predicting geomorphic response of rivers todams is an ideal experiment and learning opportunity.The questions posed by this paper are: Where do westandwithrespecttothistypeofquantitativeprediction? Is our ability to predict downstream response improvedby focusing on the effects of dams on a specific type ofchannel, those with beds primarily composed of gravel? Motivating the second question is the observation thatdespite decades of geomorphic research demonstratingthatriversoccupydistinctprocessdomainsasafunctionof where they sit in the landscape (e.g., Schumm, 1977;Montgomery, 1999), and a wide range of approaches to