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1114 Stream flow dependency ROOD, BRAATNE AND HUGHES The determination of a causal relationship between river damming and dewatering and cottonwood decline is based on evidence that cottonwoods are dependent on water originating from stream flow. There are five lines of evidence supporting this association (Table 1). Natural occurrence In semi -arid regions where low precipitation limits woody vegetation, cottonwoods are restricted to riparian zones where supplemental water is provided by a stream or river (Braatne et al. 1996). The extent of cottonwoods away from the river is re- lated to elevation rather than distance. Trees are restricted to streamside bands up to 3 to 4 in above the base stage of the river in late summer (Stromberg et al. 1996, Horton et al. 2001b). Damming and dewatering Studies along dammed rivers further indicate cottonwood de- pendency on stream flow (Table 1; Rood and Mahoney 1990, Friedman et al. 1998, Patten 1998). Cottonwood populations downstream from many dams have declined, especially in re- sponse to dewatering as a result of irrigation during the hot, dry period of mid- to late summer. The Big Lost River provides a vivid demonstration of the negative impacts of dewatering on riparian woodlands (Fig- ure 1). This river flows in braided alluvial channels in Idaho (latitude 43.60 N, longitude 113.5° W): some of the channels are used to convey irrigation water (R.M. Lundy, Big Lost Table 1. Evidence that riparian cottonwoods are dependent on stream flow River Irrigation District, unpublished report, 1993), whereas other channels are entirely dewatered during the irrigation period. Narrowleaf cottonwoods (P. angustifolia James) and sandbar willows (Salix exigua Nutt.) died along the diverted channels but continued to thrive along the flowing channels (Figure 1). Mortality occurred over about 5 years of summer dewatering, with complete mortality by 1993. The period from 1987 through 1991 included a sequence of dry years that prompted increased dewatering, which contributed to drought stress (R.M. Lundy, unpublished report, 1993). Lundy con- cluded that alluvial groundwater is reliant on stream flow and that mortality of cottonwood and willow occurs within 5 years of stream dewatering. There are several other reports of cottonwood decline fol- lowing damming, confirming dewatering as a cause of cotton- wood decline (Braatne et al. 1996, Mahoney and Rood 1998, Patten 1998). Additional factors contributing to cottonwood decline include livestock grazing and trampling, invasion by exotic weeds, an excess of beaver or large ungulates (elk), and herbicide application and drift (Braatne et al. 1996, Patten 1998). Supplemental flows Dewatering can result in cottonwood decline, whereas the pro- vision of supplemental water can promote cottonwood forests. Irrigation canals are often unlined, permitting water to infil- trate adjacent riparian zones. Additional vegetation, often in- cluding cottonwoods and willows, frequently follows along these canals. Natural channels are also used to convey irriga- tion flows. Such channels may be intermittent (irregular flows) Evidence Description References Natural occurrence In semi -arid regions such as prairies, cotton- Widespread observation woods are restricted to riparian zones, and upland areas are naturally treeless. Dewatering Depletion of alluvial groundwater as a result of Figure 1. Rood and Mahoney 1990, Smith et al. 1991, reduced stream flow following damming or Stromberg and Patten 1992, Busch and Smith 1995, diversion, groundwater pumping or interruption Rood et al. 1995, Pautou et al. 1997, Scott et al. 1999, 2000, of the stream to groundwater transfer has Somogyi et al. 1999, Horton et al. 2001 b, resulted in cottonwood decline. Amlin and Rood 2003 Supplemental water Cottonwoods colonize banks along irrigation Figure 2; widespread observation canals and become established following flow augmentation of natural channels that were previously treeless as a result of intermittent flows. Physiological correlations Morphology and physiology are correlated Stromberg and Patten 1990, 1992, 1996, Smith et al. 1991, with flow characteristics over time, across Busch and Smith 1995, Willms et al. 1998, treatments or in comparisons among streams. Scott et al. 1999, 2000, Horton et al. 2001 b, 2001 c, Amlin and Rood 2003, Cooper et al. 2003 Isotopic composition 110 /160 and 2H /'H ratios of cottonwood xylem Dawson and Ehleringer 1991, Busch et al. 1992, water are sometimes similar to groundwater Kolb et al. 1997 (Acer negundo), Snyder and Williams 2000, originating from stream water. Lambs and Berthelot 2002, Lambs et al. 2002, Cooper et al. 2003 TREE PHYSIOLOGY VOLUME 23, 2003