Riparian vegetation response to altered disturbance and stress regimes

River damming and flow regulation can alter disturbance and stress regimes that structure riparian ecosystems. We studied the Bill Williams River in western Arizona, USA, to understand dam-induced changes in channel width and in the areal extent, structure, species composition, and dynamics of woody riparian vegetation. We conducted parallel studies along a reference system, the Santa Maria River, an unregulated major tributary of the Bill Williams River. Flood magnitude on the Bill Williams River has been dramatically reduced since the closure of Alamo Dam in 1968: the 10-yr recurrence interval flood in the pre-dam era was 1397 m³/s vs. 148 m³/s post-dam. Post-dam average annual flows were higher due to increased precipitation in a few years, but increases in post-dam May-September flows are largely attributable to dam operation. An analysis of a time series of aerial photographs showed that channels along the Bill Williams River narrowed an average of 111 m (71%) between 1953 and 1987, with most narrowing occurring after dam closure. Multiple regression analysis revealed significant relationships among flood power, summer flows, intermittency (independent variables), and channel width (dependent variable). The pattern of channel width change along the unregulated Santa Maria River was different, with less narrowing between 1953 and 1987 and considerable widening between 1987 and 1992. Woody vegetation along the Bill Williams River was denser than that along the Santa Maria River (27 737 stems/ha vs. 7559 stems/ha, P = 0.005), though basal areas were similar (14.3 m²/ha vs. 10.7 m²/ha, P = 0.42). Patches dominated by the exotic Tamarix ramosissima were marginally (P = 0.05) more abundant along the Bill Williams River than along the Santa Maria River, whereas the abundance of patches dominated by the native Populus fremontii or Salix gooddingii was similar across rivers (P = 0.30). Relative to Populus and Salix, Tamarix dominates floodplain vegetation along the Bill Williams River (P < 0.0001). Most stands of the dominant pioneer trees on both rivers became established in the 1970s and 1980s. Recent seedling establishment occurred in wider bands along the Santa Maria River (15.3 m wide vs. 5.4 m wide on the Bill Williams River, P = 0.0009), likely due to larger floods and associated seedbed formation along the Santa Maria River. Seedling survival rates were generally higher along the Bill Williams River, perhaps due to higher summer flows.