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36 M, J, COHEN ET AL 35000 ~e "'0 - on '" '" 3??oo 25000 20000 15000 10000 ~ ~ Q on '" '" ~ ~ Q ~ ~ ~ ~ :g ~ ~~~'~~~~ Q on ~ ~ Q on Q ~ ~ ~ on \0 ~ Figure 1. Colorado river discharge at the Southerly International Boundary (SIB) 1910-1998 (_). Undepleted discharge (.........) reflects estimated undepleted discharge of the Colorado and Gila rivers at the SIB. Sources: measured discharge prior to 1935 from Morrison el al., 1996; measured discharge 1935-1998 from IBWC; undepleted Colorado River flow from U.S. Bureau of Reclamation; undepleted Gila River flow based on annual estimate from U.S. Bureau of Reclamation (1952). additional 1600 x 106 m3 year - I to the discharge of the Colorado River, at its confluence near Yuma, 18'7 km upstream from the Northerly International Boundary (NIB) (U.S. Bureau of Reclamation, 1952). This combined discharge flowed through the Colorado River delta and into the Upper Gulf of California, supporting tremendous biological productivity and diversity (Luecke et al., 1999). Except in years with unusually high run-off virtUally the entire flow of the Colorado is now captured and used before reachihg the river's mouth (Morrison et aI" 1996). Figure 1 compares discharge at the Southerly International Boundary (SIB) with the estimated combined discharge of the undepleted Colorado and Gila rivers. . Yet, despite reports that the delta was a dead ecosyste~ where the Colora~o RIver no longer reached the sea (Fradkin, 1981), agricultural dramage and the occaSIOnal space- building releases of Colorado River water from upstream reservoirs have prompted significant new growth of valuable native riparian and emergent wetland habitat, sup- porting the largest and most critical arid wetland in North America and sustaining avian and aquatic species of concern (Glenn et al., 1992, 1996; Luecke et al., 1999). Flood releases have also been strongly correlated with a rise in the shrimp catch in the Upper Gulf (Galindo-Bect et al., 2000), an indication of the renewed viability of an important estuary. , f th I" h d' 'd' B' The gauge at the SIB (the southernmost pomt 0 e Imltrop e IVI 109 ala California, Mexico from Arizona, U.S.A.), records discharge to the upstream extent of the delta. In 9 years within the most recent 30-year period of record (1969-1998), annual discharge at the SIB has exceeded 1000 x 106 m3. Me~n annu~1 dis~harge at tlle SIB during this period measured 2350 x 106 m3 year - 1, while median dIscharge was 190 x 106 m3 year-t (0' = 4400 X 106 m3year- I). The Colorado River discharges to the delta when either or both of the following sets of conditions are satisfied: the elevation of WATER BALANCE FOR THE COLORADO RIVER DELTA 37 Lake Mead on the Colorado River, or Painted Rock Reservoir on the Gila River, and projected run-off into that reservoir are both sufficiently high to trigger flood- control releases, and the timing and magnitude of such releases exceed the demands and diversion capacity of downstream diverters. This study provides a more robust assessment of sources and quantities of discharge to the Colorado River delta than has been published previously, offering historical and recent records of discharge at several specific locations near the SIB, agricultural drainage entering the mainstem, as well as calculated discharge at several other locations, including the Cienega de Santa Clara and the mouth of the Colorado River at the upper Gulf of California. Due to data constraints, the study is limited to the calendar year period 1992-1998. To refine the assessment, the study distinguishes between years in which flood stage (estimated by Luecke et at., 1999 as 10D-200 m3 s - I) of the Colorado River was exceeded and those years in which it was not. ce,.-' c.) c"...," "'- .) <.;;:) CD Description of the study area The Colorado River delta lies in the Sonoran desert, characterized by low precipitation (54mmyear-l) and high evaporation rates (2046mmyear-1) (IBWC, 1992-1998). This study employs the delta boundaries defined by recent literature (Valdes-Casillas et ai., 1998; Luecke et al., 1999), encompassing a land area of roughly 600 km2 along the border of the Mexican states of Baja California and Sonora. For the purposes of this study, the delta refers to the area downstream of Morelos Dam between the levees, plus the Rio Hardy wetlands northwest of the levee on the right bank, and the Cienega de Santa Clara (4200 ha) and ElIndio (1900 ha) and El Doctor (750 ha) wetlands east of the levee on the left bank (Fig. 2). The delta also commonly includes the intertidal zone along the final 19 km of the river, encompassing 440 ha (Luecke et al., 1999). Due to difficulties encountered in controlling for tidal effects, this study does not include the intertidal zone within the water balance. The foot of Morelos Dam, 1.8 km downstream of the NIB, marks the uppermost limit of the delta, which extends downstream along the limitrophe dividing Baja California from Arizona. The upstream extent of the delta, characterized by willow (Salix good- ingii) thickets and cottonwood (Populusfremontiz)-willow gallery forests (Valdes-Casil- las et al., 1998), is constrained within levees that were constructed to protect the surrounding agricultural areas from flooding. The area within the levees broadens downstream near the Colorado's confluence with the Rio Hardy, where the native riparian vegetation was supplanted by wetland vegetation and a higher proportion of non-native saltcedar (Tamarix ramosissima) (Luecke et al., 1999). Downstream of the confluence lies the intertidal zone, characterized by endemic saltgrass (Distichlis [)aZmeriz), affected by the extreme tides [amplitude > 8 m (Lavin et al., 1997)) of the Upper Gulf of California (Glenn el al., 1999). The delta also commonly includes three wetland areas distinct from the mainstem system: the Cienega de Santa Clara and EI Indio wetlands, characterized by dense stands of cattails (Typha domingensis), common reed (Phragmites australis) a,nd bulrush (Scirpus americanus) (Glenn el al., 1992), and El Doctor wetlands, supporting 29 wetland plant species (Zengel et al., 1995). The Cienega, the largest of these distinct wetlands, has o total inundated area of 12,000 ha, of which some 4200 ha are vegetated (Luecke et al., 1999; Zamora-Arroyo et al., 2001). The Cienega lies in a depression formed by the Cierro Prieto fault, in a former arm of the Colorado River (Glenn el al., 1999). In the 1970s, agricultural drainage from Mexico's Riito Drain and local artesian springs supported a smaller (200 ha) wetland at the site. Agricultural drainage discharged behind the levee on the left bank of the Colorado River supported EI Indio wetlands (Luecke et al., 1999). Artesian springs along the eastern edge of the delta sustained EI Doctor wetlands (Glenn et al., 1999).