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38 M. J. COHEN ET AL C A L FOR N A ---- --..--- ----.... -.....-- BAJA CALI ORNIA SON 0 R A i N \"'" River ~---./ Agrlcunural Drain ........ Levee "i Gaging Station 1) 5 10 15 20 25 KM .. . Figure 2. Major hydrologic features of the Colorado River delta (after IBWC, 1992-1998). The timing, quantity, and quality of discharge through the delta affect vegetation i differently. Riparian and wetland species have differing flow requirements, . both over their individual life-cycles and across species. Native riparian vegetation occurring along the Colorado River mainstem, such as Fremont cottonwoods and Goodding's willow, require overbank flooding to flush soils of accumulated salts (Glenn el al., 1998) and for seedling recruitment (Stromberg, 1993). Established cottonwood and willow seedlings depend upon the alluvial aquifer, ratheI' than directly upon instream flows (Dawson & Ehleringer, 1991; Stromberg, 1993). Emergent vegetation, such as that occurring in the Rio Hardy wetlands and the Cienega de Santa Clara, depends upon available surface water (Glenn el al., 1992, 1996). In its delta, the Colorado River was a low gradient, meandering stream with no firm channel: the downstream, broader reach of the floodplain is replete with oxbows and' backwaters, vestiges offormer channels (Sykes, 1937). The total length of the Colorado River from Morelos Dam to its mouth near Isla Montague, at the Upper Gulf of WATER BAlANCE FOR THE COLORADO RIVER DELTA 39 California, was about 150 km (C. Valdes-Casillas, pers. comm.). The maximum elevation of the river occurred at the base of Morelos Dam, with a streambed elevation approximately 32 m a.s.l. Ongoing dredging operations in the limitrophe, administered by the binational,International Boundary apd Water Commission (IBWC), will lower streambed elevation by as much as 8 m. The Rio Hardy, a former channel of the Colorado River (Sykes, 1937), runs 24 km to its mouth at the Colorado River, discharg- ing primarily agricultural drainage from the Mexicali Valley (Valdes-Casillas el al., 1998), In flood years, water drained from the mainstem below the confluence with the Rio Hardy into the Laguna Salada basin, which has a minimum elevation of - 3 m a.s.l, The flow of Colorado River water to Mexico and the delta is tightly controlled. The 1944 U,.S.-Mexico Treaty on the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande (1944 Treaty) commits the U.S. to deliver 1850 x 106 m3 of Colorado River water to Mexico each year, of which at least 1678 x 106m3 is to be delivered at the NIB and the remainder may be delivered at the SIB near the mainstem. In years in which a surplus of water exists in excess of U.S. demands, the Treaty commits the U.S. to deliver up to an additional 246 x 106 m3 of water to Mexico (Hundley, 1966). A 1973 amendment to the 1944 Treaty, and resultant federal implementing legisla- lion, led in 1977 to the discharge of brackish ( > 2900 ppm) ground-water (previously dischargedinto the mainstem) into an area in the south-eastern edge of the delta, greatly expanding the extent of the Cienega de Santa Clara from some 200 ha to an estimated 20,000 ha (Glenn el al., 1992; Zengel et aI., 1995). While the institutional context controls the timing and quantity of Colorado River discharge, the physical infrastructure in and around the delta determines the location of deliveries. This infrastructure, displayed in Fig. 2, includes Morelos Dam, a levee system, and agricultural drains and wasteways. Morelos Dam is a run-of-the-river diversion dam with no effective storage capacity. Instantaneous discharge in excess of Morelos Dam's diversion capacity of 226 m3 s - I, and discharge in excess of agricul- tural and urban consumptive use orders, pass through the dam and into the mainstem. Between Morelos Dam and the SIB, the II-Mile Wasteway (5'1 km downstream of Morelos Dam) and the 21-Mile Wasteway (28'0 km downstream) discharge agricultural drainage, from the Valley Division of the Yuma Project in Arizona, into the mainstem. On an emergency basis, Wellton-Mohawk drainage water has been discharged to a point immediJltely below Morelos Dam (IBWC, 1992-1998). The mwc measures combined mainstem discharge at the SIB (IBWC, 1992-1998). These records reflect discharge at the upstream boundary of the Colorado River mainstem portion of the delta (Glenn el aI., 1996; Luecke el aI., 1999), and have been used in other studies as a proxy for discharge into the Upper Gulf of California (Lavin & Sanchez, 1999; Galindo-Bect et al., 2000). Colorado River flow at the SIB over the period of record (1935-1998) was highly variable (mean = 3272 x 106m3 year-I; me- dian = 1237 x 106m3year-1; u = 4301 x 106m3 year-I). Some of the water Mexico diverts at Morelos Dam is conveyed via the Central Feeder Canal to a point 5 km downstream from the SIB, where it may be returned to the river via the KM 27 Wasteway on the right bank of the river or may be diverted to the Bacanora-Monumentos Canal system via the Sanchez Mejorada siphon, to irrigate fields in the San Luis Valley on the left bank of the river. Municipal effluent from the City of San Luis Rio Colorado discharges to the Colorado River on the left bank of the river, near KM 27. The KM 38 Wasteway, 45'3 km downstream from the SIB and 1.3 km upstream from the railroad bridge, returns water from the Barrote Canal to the main- stem (IBWC, 1992-1998), The Carranza and Principal Southern drains discharge directly, via gates in the levee, into the Colorado River, while the Nayarit, Cucapa, and Southern Collector drains discharge into the Rio Hardy. The Riito drain discharges into the Cienega, while the Plan de Ayala drain discharges into Ei Indio wetlands. The Main Outlet Drain Bypass Extension (Bypass Extension), diverting brackish ( >3200 ppm) c;:. c.::> c,....') ['..) o o