Instream flows and the decline of riparian cottonwoods along the Yakima River, Washington, USA

Abstract

Under pre-settlement conditions the Yakima River in Washington state, USA was characterized by multiple channels, complex aquifers and extensive riparian cottonwood forests. Subsequent implementation of headwater dams to supply irrigation water has altered river and floodplain processes critical to the cottonwoods and associated riparian vegetation. In this study, we analysed hydrology and floodplain forests and especially the dominant black cottonwoods (Populus trichocarpa) along sequential reaches of the Yakima River. Elevations were surveyed and vegetation inventoried along cross-sectional belt transects, and cottonwood tree ring interpretations investigated historic associations between river hydrology and cottonwood establishment and growth. We analysed hydrographs relative to the apparent episodes of cottonwood recruitment and applied a quantitative model for seedling colonization that required: (1) floods, disturbance flows to produce barren nursery sites, and subsequent flows for seedling (2) establishment and (3) survival. In contrast to earlier conditions, flow patterns after the 1960s have generally been unfavourable for cottonwood recruitment although some cottonwood colonization has occurred in association with physical disturbance from gravel mining. With recent flow regimes, regulated flows along upper reaches maintain the river near bank-full throughout the growing season, thus inundating suitable seedling recruitment sites. Downstream, irrigation withdrawals reduce the river stage, resulting in seedling establishment at low elevations that are lethally scoured by subsequent high flows. These regulated flow regimes have not hindered growth of established trees, but have reduced the recruitment of cottonwoods, and particularly disfavoured females, thus altering sex ratios and producing skewed cottonwood population age and gender structures. The cottonwood decline has also been associated with other changes in riparian plant community composition, including the encroachment of invasive weeds. Based on this ecohydrologic analysis we discuss flow adjustments that could rejuvenate cottonwood forests along the Yakima River.

The influence of hydrological regimes on sex ratios and spatial segregation of the sexes in two dioecious riparian shrub species in northern Sweden

“…In the Ume River system, in contrast, stress from water tables that remain constantly high during the growing season may be causing long-term reduction in the resilience of S. myrsinifolia−phylicifolia individuals though not, it would seem, through mechanisms that have, so far, caused a shift in sex ratios. This is in contrast to the findings of Braatne et al (2007) along the Yakima River in Washington (US) who correlated male dominated sex ratios in young P. trichocarpa Torr. and A.…”

Population structure, physiology and ecohydrological impacts of dioecious riparian tree species of western North America

“…As with A. negundo, skewed sex ratios have been observed in many populations of Populus (Grant and Mitton 1979;Comtois et al 1986;Braatne et al 1996;Gom and Rood 1999;Braatne et al 2007), but not all (Kaul and Kaul 1984;Stanton and Villar 1996;Rowland and Jackson 2001). In cases where sex ratios occur, males are relegated to more extreme environments (Braatne et al 1996;Rowland and Jackson 2001).…”

Assessment Approach for Identifying Compatibility of Restoration Projects with Geomorphic and Flooding Processes in Gravel Bed Rivers

“…For example, in the Wapato Reach of the Yakima River, the data indicated that the preflow regulation bankfull discharge was sufficiently large to maintain a braided state for the given gradient (Leopold and Wolman 1957), but the current flow regime was more likely to be associated with a meandering or transitional planform (Ferguson 1984;van den Berg 1995). However, flow regulation may be promoting a braided state because of the associated impacts of reduced flow on growth and persistence of floodplain vegetation (Braatne et al 2007) that otherwise provides bank stability (Millar 2000). This outcome suggested that actions leading to development of more stable islands and bars through structures that enhance floodplain deposition and protect revegetation areas could eventually lead to a single dominant, narrower, deeper channel under the present regulated flow regime.…”