Importance of low‐flow and high‐flow characteristics to restoration of riparian vegetation along rivers in arid south‐western United States

Abstract

Environmental Services, Salt River Project, Tempe, AZ, U.S.A. SUMMARY 1. Riparian vegetation in dry regions is influenced by low-flow and high-flow components of the surface and groundwater flow regimes. The duration of no-flow periods in the surface stream controls vegetation structure along the low-flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow-regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low-flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought-tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient-holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood-dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on floodsuppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 651 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south-western United States, some focus on re-establishing hydrogeomorphic processes by restoring appropriate flows of surface water,...

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

“…Decreased baseflow and drawdown of groundwater levels has been associated with a shift in dominance to xerophytic species in the American Southwest at the expense of forbs and obligate phreatophytes (Stromberg et al, 1996(Stromberg et al, , 2006(Stromberg et al, , 2007(Stromberg et al, , 2010. Xerophytes in the riparian corridors of the American Southwest include deep-rooted phreatophytes (e.g.…”

Changes in Community-Level Riparian Plant Traits over Inundation Gradients, Colorado River, Grand Canyon

“…Disturbance and abiotic stress are well-known limitations on plant cover (Grime 1977;Stromberg et al 2007). In our study of riparian vegetation in Grand Canyon, the primary disturbances are shear stress, scouring, abrasion, and burial from deposition during floods, and the primary abiotic stresses include seasonal submergence and inundated anaerobic soils near the river, and drought stress at more upland sites within the river corridor (Howard and Dolan 1981;Stevens et al 1995;Sankey et al 2015).…”

“…In riparian ecosystems of arid regions, flow regime drives physiological drought and inundation stresses and physical flooding disturbance (Naiman et al 1993;Lytle and Poff 2004;Stromberg et al 2007). Gradients of disturbance and water availability strongly determine the composition and location of vegetation communities in riparian ecosystems (Pockman and Sperry 2000;Nilsson and Svedmark 2002;Sluis and Tandarich 2004).…”

Groundwater circulation relative to water quality and vegetation in an arid transitional zone linking oasis, desert and river

“…Groundwater in oasis and riparian zone is charactered by modern age, shallow depth, and low salinity. The study on the relationship between vegetation coverage and groundwater circulation is of great importance for recovering degraded ecosystem and managing groundwater dependent ecosystems in the (semi)arid areas, such as other arid areas in NW China [53], southwestern United States [54] and Australia [55,56].…”