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groundwater supplies that it can pump in the border region, while California and Nevada have proposed an action - the lining of the All American Canal in Califor- nia-that would substantially capture water that now recharges this groundwater basin. Given the relative scarcity of water in the Colorado River basin, it is more likely that the delta's salvation will occur through some level of protection for flows that are presently occurring but are not mandated. This report focuses on steps that can be taken to improve the management of existing flows, rather than steps to increase transboundary water deliveries. In the short term, delta ecosystems will continue their recovery if flows occur at levels recorded in recent years. However, some assurance of these flows, as well as dedicated trans-boundary water deliveries, may be part of the long-term solution. APPORTIONED FRESHWATER FLOWS When Colorado River waters were apportioned, first by the 1922 compact and subsequently an Upper Basin compact, court decisions, federal law, and international treaty, the river was overallocated. The problems arising from this over allocation are compounded by the fact that there are very different interpretations of the definition of consumptive use, treatment of evaporation from reservoir surfaces, and the water delivery obligations of the Upper Basin states under the treaty to Mexico (Getches, 1985) If the Colorado is already ovemllocated because water entitlements were based on optimistic estimations of average annual flow, it may be all the more difficult to secure additional water allocations dedicated to delta ecosystems. This reinforces the importance of finding ways to improve management of existing flows that now reach the delta. In any case, cooperation, accommoda- tion, and creativity will be essential, especially as demands for water increase. Before 1980, while major reservoirs on the Colorado River were still filling, flood flows were nonexistent. The riparian zone of the river from Morelos Dam to the junc- tion with the Rio Hardy was a dry ecosystem, dominated by widely spaced mesquite trees (Valdes-Casillas et aI., 1998a).21 Below the junction of the two rivers, the channel was perennial, due to the discharge of agricul- tural drain water from the Mexicali Valley and tidewater entering from the Gulf of California. In the years since 1980, flood flows have been released from Lake Mead (the last major storage on the Colorado River in the U.S.) when flow exceeds storage capacity and upstream use. In years without flooding, the only Colorado River water to reach Mexico is its 1.5 maf (1.8 x 109 m3) treaty allotment, about 10 percent of the river's average annual flow.22 The U.S. delivers nearly all of Mexico's water allotment to the Northern International Boundary at Morelos Dam. Mexico diverts this water to the Mexicali and San Luis Rio Colorado irrigation districts by way of the Central Canal,23 which has a capacity sufficient to divert Mexico's entire allo- cation. Water in the Central Canal not used for irrigation is routed to Mexicali and Tijuana for municipal use (Ybarra, 1999). The relatively small portion of Mexico's allocation that is delivered at the Southern International Boundary at San Luis Rio Colorado is also diverted for irrigation. During flood-free years, no Colorado River water reaches the remnant delta wetlands below the irrigated farmland (Glenn et aI., 1996).24 The only flows that continue past Mexico's irrigation diversions-and into the delta-are flood flows (see note 2 and "Tides and Floods" beloW). During dry years, the only water reaching the delta comes from groundwater seeps, agricultural drainage, and tidewater. [See Box 1.] 20 Mexico has objected to the quality of this water, stating that it is so saline (averaging 1500 parts per million [ppm]) that it reduces yields on the 94,000 acres (38,000 ha) where it is used for irrigation, causing soil deterioration and increasing groundwater salinity. Additionally, Mexico has suggested that this water is possibly contaminated with pesticides. Mexico has requested that its entire entitlement be delivered to the Northern International Boundary (NIB) at Morelos Dam on the river's main channel. The U.S.-Mexico agreement concerning water quality in the Colorado River is found in Minute 242 to the 1944 Treaty. It requires !hat the average annual salinity of water delivered at the NIB not exceed 115 +/-30 ppm over the annual average salinity of water arriving at Imperial Dam in California (Pontius, 1997). A portion of Mexico's treaty water is sent to the Southern International Boundary (SIB), where agricultural wastewater collected by several drains, and at times augmented by pumped groundwater, is delivered, 2 miles east of the river channel. Water quality at the SIB is not governed by the treaty and has an average salinity of 1500 ppm. Approximately 140,000 acre-feet (1.7 x 10. m3) of Mexico's 1.5 maf (1.8 x 109 m3) entitlement is delivered to the (SIB). The salinity of seawater is 35,000 ppm. 21 These observations are based on inspection of 1972 aerial photographs and interviews with residents. 22 The average flow (over the historic long term) of the Colorado River Is 15 maf (1.8 x 10'0 m3). Flows as low as 6 maf (7.4 x 109 m3) and as high as 24 maf (3 x 10'Om 3) have been recorded (Pontius, 1997). 23 Approximately 2 maf (2.4 x 109 m3) per year are used for Irrigation in the Mexicali and San Luis Rio Colorado valleys, with Colorado River water making up the majority of this supply (Valdes-Casillas et aI., 1998a). 24 While the Law of the River gives clear priority to consumptive uses of water, the Colorado River also provides considerable value in terms of recreational and fish and wildlife benefits. However, the ecological needs of the Colorado River have only recently gained legal recognition and protection. Some of these conservation efforts in the U.S. are discussed in Chapter 1 . 12