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~,; ~J , :,1 ;~ ..;;, As noted in Table 2, these two tributaries have provided an average annual inflow of over 600,000 acre-feet during the indicated periods of record. During the past 10-15 years, however, their combined annual inflow has been on the order of 500,000 acre-feet. The seasonal distribution of these tribu- tary inflows is illustrated in Figure 3 for WAU 130403 (Devils) and WAU 130700 (Pecos). The average annual release from Amistad Reservoir into WAU 130800 since completion of the impoundment in 1968 has been about 1,500,000 acre-feet [4]. ,', ;.: " Downstream of Amistad Reservoir in WAU's 130800 and 130900, flows in the River are regulated and controlled for both flood control and water supply , purposes. Over two million acre-feet of storage are provided in Amistad Reservoir for flood control, and Falcon Reservoir, at the lower end of WAU 130800, provides an additional 1.3 million acre-feet of floodwater storage capacity [4]. Flood protection levees have been constructed along the length of the mainstem channel from Mission, Texas, downstream to below Brownsville. Floodwaters in the mainstem channel are diverted around the lower Rio Grande valley through floodways on both sides of the River. Each floodway system has a capacity of 105,000 cfs (cubic feet per second), with the mainstem channel designed to carry 20,000 cfs through the lower valley di rectly to the Gulf [16]. Normal and low flow releases from both Amistad and Falcon Reservoirs provide water supplies for municipal, industrial and irrigation uses in both the United States and Mexico. Diversions for municipal and industrial pur- poses average about 65,000 acre-feet annually (1968-1978 period of record) [4]. Over two million acre-feet are diverted annually for irrigation, of which slightly more than half is for agriculture in the United States. Of the United States portion, about one million acre-feet [17] are used annually for irrigation outside the Region, but still within the lower Rio Grande valley, i.e. outside the flood control levees. Most of the return flows associated with these diversions are discharged into drains and f100dways which do not flow back to the mainstem of the River. As indicated in Table 2, the mean annual flow of the Rio Grande below Falcon Dam is about 2,300,000 acre-feet '(1954-1978), whereas the mean annual discharge of the River directly to the Gulf below Brownsvi11e is only about 900,000 acre-feet. 17 . . r r J ," I" " '. \.'..... {..,...,..