Otir Liviiii^ HcMiimt's — Ripmian Ecn.wslcitts 



287 



aspects of How ihal promole successful estab- 

 lishment. We deleniiineti the precise age and 

 elevation of establishment of 151 plains cotton- 

 wood stems in the study site and related their 

 years of establishment to the How record from 

 U.S. Geological Survey (USGS) gauges. 



Lower Colorado River 



We established three sites for an intensive 

 ecophysiological analysis of riparian plant com- 

 munities on the Havasu National Wildlife 

 Refuge in the lower Colorado River floodplain. 

 Our analyses were confined to stands of ripari- 

 an vegetation that had been classified as cotton- 

 wood-willow habitats (Anderson and Ohmart 

 1984; Younker and Andersen 1986). 



Hydrology and Riparian 

 Ecosystem Dynamics 



Reproduction and growth of riparian plant 

 species are closely associated with peak flows 

 and related channel processes such as meander- 

 ing. Successful establishment of such plants 

 typically occurs only in channel positions that 

 are moist, bare, and protected from removal by 

 subsequent disturbance (Sigafoos 1964; Everitt 

 1968; Noble 1979; Bradley and Smith 1986; 

 Stromberg et al. 1991; Sacchi and Price 1992; 

 Johnson 1994). If streamflow is diverted, young 

 trees may die (Smith et al. 1991). Studies of 

 plant water uptake in floodplain ecosystems 

 indicate that maintenance of cottonwood and 

 willow populations depends on groundwater 

 moisture sources which, in turn, are closely 

 linked to instream flows (Busch et al. 1992). 

 Thus, the establishment and maintenance of 

 riparian plant communities are a function of the 

 interplay among surface water dynamics, 

 groundwater, and river channel processes. 



Maps and notes from the journals of Lewis 

 and Clark (1804-06) suggest that the present 

 distribution and abundance of cottonwoods 

 along the Missouri River within the study reach 

 are generally similar to presettlement condi- 

 tions. Although flows through this reach are 

 influenced by Canyon Feiry Dam on the main- 

 stem and Tiber Dam on the Marias River, the 

 gross seasonal timing of flows and the magni- 

 tude and frequency of daily maximum flows 

 have not been greatly altered by dam opera- 

 tions. This is due in part to the dam's relatively 

 small storage capacity and the presence of a 

 number of unregulated tributaries below the 

 dams. Thus, the study reach represents one, if 

 not the last, semi-naturally functioning reach 

 along the entire Missouri River. 



In the Colorado River, the link of floodplain 

 groundwater with instream flows is illustrated 

 by the association of river discharge and fluctu- 

 ations in water table depth in the adjacent 



floodplain (Fig. 2). Further evidence for this 

 linkage comes from daily fluctuations in water 

 table depth, which coiTelated closely with the 

 Colorado River hydroperiod (Busch. unpub- 

 lished data). Colorado River floodplain soils 

 were dry. Volumetric soil nmisture in the upper 

 1 m (3.3 ft) of the Colorado River soil prollle 

 averaged less than 4%. while that of the nearby 

 and less heavily impacted Bill Williams River 

 averaged 13*7^. Incision of stream channels, 

 through either natural or human-induced caus- 

 es, can lead to the depression of floodplain 

 water tables (Williams and Wolman 1984). 

 Channelization of the lower Colorado River 

 appears to have led to floodplain groundwater 

 declines, and this has tended to isolate riparian 

 vegetation from its principal moisture source at 

 or near the water table (Busch et al. 1992). 



Salinity and Alteration of 

 Riparian Ecosystem Processes 



In regulated rivers, a lack of flooding or 

 infrequent groundwater incursion into surface 

 soils can result in altered nutrient dynamics. 

 The lack of an aqueous medium for salt disper- 

 sal may result in the elevation of soil salinity to 

 levels that are stressful to some of the trees and 

 shrubs native to southwestern riparian ecosys- 

 tems (Busch and Smith 1995). Colorado River 

 soils were significantly (P < 0.05) more saline 

 than soils in the adjacent Bill Williams River 

 floodplain. Salinities in Colorado River soils 

 exceeded levels shown to inhibit germination, 

 reduce vigor, and induce mortality in seedling 

 cottonwood and willow (Jackson et al. 1990). 

 Salt-tolerant species could thus benefit from 

 elevated alluvium salinity. Evidence for salinity 

 tolerance in both native and exotic halophytes 

 (plants growing in salty soils or a saltwater 

 environment) shows that arrowweed [Tessaria 

 scricea) and tamarisk had significantly (P < 

 0.05) higher leaf tissue sodium concentrations 

 (11.2 and 18.1 mg/g (ppt], respectively) than 

 did cottonwood (1.1 mg/g [pptj) and willow 

 (0.7 mg/g [ppt]). 



JFMAMJJASOIMD 

 Month 



Fig. 2. Streamllow in the lower 

 Colorado River and water table 

 depth fluctuations in the adjacent 

 floodplain. 



Colorado River study site with 

 willow (note stress-induced 

 canopy die back) and exotic 

 tamarisk. 



I Constrained reach of the Missouri 

 S River. Montana. 



