2S6 



Riparian Ecosystems — Our Living Resources 



Western 



Riparian 



Ecosystems 



hy 



David E. Biisch 

 National Park Senice 



Michael L. Scott 

 National Biological Service 



In much of western North America, riparian 

 (streamside) environments are the only pail of 

 the landscape moist enough to allow survival of 

 trees (Fig. 1). Riparian landscapes are usually 

 defined as ecotones or corridors between terres- 

 trial and aquatic realms (Malanson 1993). In 

 spite of their limited areal extent, riparian 

 ecosystems are essential habitat for many verte- 

 brate species and provide critical physical and 

 biological linkages between terrestrial and 

 aquatic environments (Gregory et al. 1991). 



Because of their association with scarce sur- 

 face water resources, western riparian ecosys- 

 tems have long been influenced by human activ- 

 ities. Human-caused perturbations can alter 

 energy and material flow in riverine ecosys- 

 tems, thus modifying riparian plant communi- 

 ties (Brinson 1990). Among the most serious 

 impacts to riparian ecosystems are water 

 impt)undmcnt and diversion, groundwater 

 pumping from alluvial aquifers, livestock graz- 

 ing, land clearing for agriculture or to increase 

 water yield, mining, road development, heavy 

 recreational demand, fire, the elimination of 

 native organisms (e.g., beaver [Caslur canaden- 

 sis]) or the introduction of exotics, and overall 

 watershed deeradation (Stromberc 1993). 



Fig. 1. A cottonwood-willow riparian ecosystem illustrat- 

 ing how trees are closely associated with a water source in 

 an and landscape. Arikaree River. Colorado. 



Riparian ecosystems along most major west- 

 em rivers have changed as the result of water 

 development and flood control. Losses of ripar- 

 ian forest downstream of dams have been 

 reported from throughout western North 

 America (Rood and Mahoney 1990). In con- 

 trast, woodland expansion in other dam-regulat- 

 ed riparian ecosystems provides evidence that 

 the inteiTelationships between plant communi- 

 ties and hydrogeomorphic processes are com- 

 plex (Johnson 1994). As the result of wide- 

 spread, human-induced changes in hydrology 

 and land use. native cottonwood-willow stands 

 are being replaced by non-native woody species 

 such as Russian olive (Elaeagmis angustifolia) 

 and tamarisk (Tamarix mmosissima) throush- 



out the West (Olson and Knopf 1986; Knopf 

 and Scott 1990; Stromberg 1993). 



In this article, we contrast the roles played 

 by natural and human-induced disturbances in 

 structuring western riparian ecosystems. Our 

 approach draws heavily on data from the lower 

 Colorado and upper Missouri rivers, two large, 

 diverse systems that showcase a range of natur- 

 al and human factors intlueiicing riparian 

 ecosystems throughout western Noilh America. 

 We also focus on how water and land-use man- 

 agement may threaten these valuable ecological 

 resources. 



Most of the Missouri Rivet, through the 

 Dakotas to its confluence with the Mississippi 

 River, is controlled by a series of large dams 

 and reservoirs constructed between the 1930's 

 and 1950's. These dams radically altered the 

 magnitude, timing, and frequency of flood 

 flows that formerly promoted regeneration and 

 maintenance of extensive riparian cottonwood 

 {Popiilus (leltoides) forests (Johnson et al. 1976; 

 Johnson 1992). Here, we examine the impor- 

 tance of flow variability and channel processes 

 in creating and maintaining riparian cotton- 

 wood stands in one of the last relatively natural- 

 ly functioning reaches of the upper Missouri 

 River in Montana. 



The lower Colorado River riparian ecosys- 

 tem (Nevada. California, and Arizona) has also 

 been affected by hydrologic change resulting 

 from human activities. Declines in riparian for- 

 est dominated by cottonwood {Populus frenum- 

 tii) and willow {Sali.x gooddingii) have been 

 attributed to change in the physical environment 

 and to the extensive invasion of tamarisk. Our 

 evaluation of the Colorado River ecosystem 

 centered on an investigation of surface water- 

 groundwater linkages and how hydrologic fac- 

 tors affect water uptake and use in riparian trees 

 and shrubs. We also examined how hydrologic 

 perturbation and alteration of natural distur- 

 bance processes affect riparian community 

 structure along the lower Colorado River. 



Methods 



Upper Missouri River 



We intensively sampled nine sites between 

 Fort Benton, Montana, and Fort Peck Reservoir. 

 Sites were selected primarily to represent the 

 range of geomoiphic {see glossary) conditions 

 observed within this reach. Channel movement 

 is variously constrained through this portion of 

 river; in some reaches the channel meanders 

 whereas in other reaches lateral movement is 

 limited to a narrow valley floor. Previous work 

 demonstrates that the age structure of cotton- 

 wood populations is strongly influenced by 



