Issues in Ecology Number 10 Winter 2003 Human alterations of river flow have seldom taken BOX 1- DEFINING FLOW CONDITIONS FOR RIVERS AND STREAMS into account the ecological consequences."Many rivers now resemble elaborate plumbing works,with the timing and Base flow conditions characterize neriods of low flov amount of flow completely controlled.like water from a faucet hotwoon storms They define the minimum quantity so as to maximize the rivers'benefits for humans,"wrote of water in the channel.,which directly influences water policy expert Sandra L.Postel."But while modern habitat availability for aquatic organisms as well as engineering has been remarkably successful at getting wate the depth to saturated soil for riparian species.The to people and farms when and where they need it,it has magnitude and duration of hase flow varies areatly failed to protect the fundamental ecological function of rivers among different rivers,reflecting differences in climate and aquatic systems. geology.and vegetation in a watershed. in the U..West are prime Frequent(that is,two-year return interval)floods human manipul tion o reset the system by flus damages to riverban streambed,thus promoting higher production during nming rivers a and da base flow period High flows may also facilitate dispersal o organis hghf6anudo contribute 0 e ate which used to bopsanadmahpaianegeatondynanie e adja support diverse riparian Sediment and Organic Matter Inputs tant reformative events for systems They transnort large amounts of sedimer In river systems the move ement of sediments and often transferring it from the main channel to influxes of organic matter are impd ortantc ponents of habita floodolains Habitat diversity within the river structure and dynamics.Natural on ganic matter inputs include increased as channels are scoured and reformed and seasonal runoff and debris such as leaves and decaying plan successional dynamics in riparian communities and material from land-based communities in the watershed floodplain wetlands are reset. Large flows can also Especially in smaller rivers and streams.the organic matte remove species that are poorly adapted to dynamic that arrives from the land is a particularly important source river environments such as upland tree species or non of eneray and nutrients.and tree trunks and other woody native tish species.The success of non-native invaders materials that fall into the water provide important substrates is often minimized by natural high flows. and the and habitats for aquatic organisms.Natural sediment restriction of major floods by reservoirs plays ar movements are those that accompany natural variations in important role in the establishment and proliferation water flows In lakes and wetlands.all but the finest inflowind of exotic species in many river systems. sediment falls permanently to the bottom,so that over tim Seasonal timing of flows, especially high flows, these systems fill.The invertebrates,algae,bryophytes ritical for d to epro e high flow runs Alon rs cotto ood trees seeds during peak snowmelt to maximize the negative consequences for aquatic and ripariar communities Annual variation in flow is an important factor influencing river systems.For example,year-to-year variation in runoff volume can maintain high species diversity.Similarly.ecosystem productivity and foodweb structure can fluctuate in response to this This variation also ensures Figure 3-Livestock use of streams can have impacts on speci n different years.thus the amount of sediment and nutrients inputs.Photo cour promoing iversity. tesy the U.S.Geological Survey.South Platte National Water Quality Assessment Program (NAWOA). 5 Issues in Ecology Number 10 Winter 2003 Human alterations of river flow have seldom taken into account the ecological consequences. “Many rivers now resemble elaborate plumbing works, with the timing and amount of flow completely controlled, like water from a faucet, so as to maximize the rivers’ benefits for humans,” wrote water policy expert Sandra L. Postel. “But while modern engineering has been remarkably successful at getting water to people and farms when and where they need it, it has failed to protect the fundamental ecological function of rivers and aquatic systems.” Rivers in the U.S. West are prime examples of how human manipulation of water flows can lead to multiple damages to riverbank and floodplain processes and communities. Damming rivers and dampening natural variations in flow rates by maintaining minimum flows year round have contributed to widespread loss of native fish species and regeneration failure of native cottonwood trees, which used to support diverse riparian communities (BOX 2). Sediment and Organic Matter Inputs In river systems, the movement of sediments and influxes of organic matter are important components of habitat structure and dynamics. Natural organic matter inputs include seasonal runoff and debris such as leaves and decaying plant material from land-based communities in the watershed. Especially in smaller rivers and streams, the organic matter that arrives from the land is a particularly important source of energy and nutrients, and tree trunks and other woody materials that fall into the water provide important substrates and habitats for aquatic organisms. Natural sediment movements are those that accompany natural variations in water flows. In lakes and wetlands, all but the finest inflowing sediment falls permanently to the bottom, so that over time these systems fill. The invertebrates, algae, bryophytes, BOX 1— DEFINING FLOW CONDITIONS FOR RIVERS AND STREAMS Base flow conditions characterize periods of low flow between storms. They define the minimum quantity of water in the channel, which directly influences habitat availability for aquatic organisms as well as the depth to saturated soil for riparian species. The magnitude and duration of base flow varies greatly among different rivers, reflecting differences in climate, geology, and vegetation in a watershed. Frequent (that is, two-year return interval) floods reset the system by flushing fine materials from the streambed, thus promoting higher production during base flow periods. High flows may also facilitate dispersal of organisms both up- and downstream. In many cases moderately high flows inundate adjacent floodplains and maintain riparian vegetation dynamics. Rare or extreme events such as 50- or 100-year floods represent important reformative events for river systems. They transport large amounts of sediment, often transferring it from the main channel to floodplains. Habitat diversity within the river is increased as channels are scoured and reformed and successional dynamics in riparian communities and floodplain wetlands are reset. Large flows can also remove species that are poorly adapted to dynamic river environments such as upland tree species or non￾native fish species. The success of non-native invaders is often minimized by natural high flows, and the restriction of major floods by reservoirs plays an important role in the establishment and proliferation of exotic species in many river systems. Seasonal timing of flows, especially high flows, is critical for maintaining many native species whose reproductive strategies are tied to such flows. For example, some fish use high flows to initiate spawning runs. Along western rivers, cottonwood trees release seeds during peak snowmelt to maximize the opportunity for seedling establishment on floodplains. Changing the seasonal timing of flows has severe negative consequences for aquatic and riparian communities. Annual variation in flow is an important factor influencing river systems. For example, year-to-year variation in runoff volume can maintain high species diversity. Similarly, ecosystem productivity and foodweb structure can fluctuate in response to this year-to-year variation. This variation also ensures that various species benefit in different years, thus promoting high biological diversity. Figure 3—Livestock use of streams can have impacts on the amount of sediment and nutrients inputs. Photo cour￾tesy the U.S. Geological Survey, South Platte National Water Quality Assessment Program (NAWQA)