Dolores River Core Science Report

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Fluvial disturbance associated with high flows serves to maintain heterogeneity in riparian habitats by influencing the distribution and characteristics of sediment deposits in the channel and across the floodplain. The heterogeneous habitat maintained by fluvial processes contributes to the high species richness in riparian areas (Naiman et al. 1993, Tiegs et al. 2005). Because large (i.e., overbanlc) floods do most of the geomorphic work along rivers, many of the characteristics of the physical habitat --such as the shape of the channel and the particle size of the substrate fornung the bed and banks of the stream-- are structured to some degree by flooding. One large flood can create habitat that is maintained by smaller floods and serves as sites of plant colonization for decades (Baker 1988, Friedman and Lee 2002, Cooper et al. 2003, Lytle and Merritt 2004). The texture of sediment deposited by floods influences water availability to plants as it governs the water holding capacity of the substrate. Flooding also results in the removal or burial of plants or portions of plants and results in open regeneration sites for pioneer species to colonize. High flows may also prevent encroaclunent of vegetation into the channel (Friedman and Auble 1999). Flooding also recharges floodplain soils and shallow alluvial aquifers, facilitates decomposition of leaf litter and the availability of nutrients on the floodplain, and increases the productivity of riparian vegetation (Merritt and Cooper 2000, Ellis et al. 1998). Cottonwood (Po~trltrs delto~des and P. a~rgrrs~tifolia) are valued riparian forest species. Cottonwood colonize recently disturbed open patches of moist sediment. The processes that form the types of sites that cottonwood can become established upon include those associated with flooding, such as overbanlc sediment deposits and point bar development associated with lateral channel migration (Bradley and Smith 1984, Scott et al. 1996). Channel narrowing can also facilitate or be facilitated by cottonwood establishment (Friedman et al. 1996, Scott et al. 1996) or establishment of other species such as tamarisk (Graf 1978, Allred and Schnudt 1999). Reducing the frequency of high flows can cause shifts in species composition in riparian areas by favoring those species that are more aggressive competitors (i.e., some non-native plant species) and causing a reduction in those species adapted or otherwise able to colonize following such disturbances. One prominent example is the widespread decline of cottonwood forests throughout western North America in the 20th cent<iry (Rood and Mahoney 1990). Reduction in fluvial processes (i.e., overbanlc flooding, lateral channel migration) associated with reductions in peak flows due to water storage has been implicated in reduced recn~itment and reductions in extent of cottonwood forests (Bradley and Smith 1986, Rood and Mahoney 1990, Cooper et al. 1999, Braatne et al. 1996). There are cases of cottonwood forests temporarily expanding in extent in response to river damming in areas where shallow groundwater facilitated colonization and channel narrowing (Johnson 1994). Often, reductions in high flow initiate a directional vegetation response: shifts in zonation of vegetation toward the channel. Reductions in the frequency of flooding can also result in other indirect changes such as an accumulation of salts in floodplain soils. Over long periods of time, the lack of occasional flooding can result in significant shifts in species composition of riparian vegetation due to changes in biological, physical, and chemical processes on the floodplain. High flows are an important flow component for riparian vegetation in that they structure physical habitat, create open sites, facilitate recruitment, maintain active chaiulels free of 45