Mortality of riparian box elder from sediment mobilization and extended inundation

To explore how high flows limit the streamward extent of riparian vegetatio n we quantified the effects of sediment mobilization and extended inundatio n on box elder (Acer negundo) saplings along the cobble-bed Gunnison River in Black Canyon of the Gunnison National Monument, Colorado, USA. We counte d and aged box elders in 144 plots of 37.2 m(2), and combined a hydraulic m odel with the hydrologic record to determine the maximum shear stress and n umber of growing-season days inundated for each plot in each year of the re cord. We quantified the effects of the two mortality factors by calculating the extreme values survived during the lifetime of trees sampled in 1994 a nd by recounting box elders in the plots following a high flow in 1995. Bot h mortality factors can be modeled as threshold functions; box elders are k illed either by inundation for more than 85 days during the growing season or by shear stress that exceeds the critical value for mobilization of the underlying sediment particles. Construction of upstream reservoirs in the 1 960s and 1970s reduced the proportion of the canyon bottom annually cleared of box elders by high flows. Furthermore, because the dams decreased the m agnitude of high flows more than their duration, flow regulation has decrea sed the importance of sediment mobilization relative to extended inundation . We use the threshold functions and cross-section data to develop a respon se surface predicting the proportion of the canyon bottom cleared at any co mbination of flow magnitude and duration. This response surface allows vege tation removal to be incorporated into quantitative multi-objective water m anagement decisions. Copyright (C) 1999 John Wiley & Sons, Ltd.