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830 NATURAL RESOURCES JOURNAL [Vol. 40 Fall 2000] MANAGING ECOSYSTEM CONSERVATION 831 stopover along the Pacific Flyway and supports large numbers of wintering waterfowl.39 Although resident and migratory bird densities have not been studied extensively, the Delta is considered a key element of the Flyway, and the only significant freshwater wetland among the Mexican Pacific Coast marshes.40 In the United States, the tot~l acreage of habitat in the lower basin of the Colorado River is estimated to support fewer than half as many birds.41 Agricultural wastewater, tides, a small amount of naturally occurring run-off, and artesian springs provide perennial water to the Delta. Seventeen agricultural drains from the Mexicali Valley flow into the Colorado River delta. The Cienega de Santa Clara42 receives agricultural wastewater from both Mexico and the United States. The U.S. agricultural wastewater flows from southemArizona's Wellton-Mohawk Irrigation and Drainage District via a canal built by the u.s. BOR in 1977, the Main Outlet Drain Extension (MODE) cana1.43 In sum, agricultural drain flows contribute an average annual volume of 200,000 acre-feet of water to the Delta.44 Flood flows along the river's mainstem sustain the increasingly rare, and ecologically valuable, native riparian vegetation in the upper reaches of the Delta. Since 1980, Colorado River flood flows have again reached the Delta intermittently due to near capacity storage at Lake Mead and a series of years with above average precipitation. From 1980 to 1993, average annual flood flows across the border (cross-border flows minus Mexico's allohnent) were 3.9 million acre-feet, nearly three times Mexico~s treaty allotment, and 25 percent of the average flow before dams blocked the river.4S In addition to freshwater flood flows, large tides flood some 81,500 acres in the Delta on a daily basis.46 In recent years, researchers inventoried the vegetative response to floods, and conduded that the reestablishment of native forest species in the riparian corridor has been a direct consequence of overbank flooding below Morelos Dam since the filling of Lake Powell!7 Specifically, modest annual flows (below Morelos Dam) of 32,000 acre-feet have been estimated to be sufficient to maintain, even improve, cottonwood-willow habitat in the upper reaches of the Delta.48 Annual flood events are not necessary for survival of these native tree species: they are capable of surviving at least a three-to-four-year interval between major flow events in the Delta floodplain.49 Pulse flows of 260,000 acre.,.feet, released at a rate of3~7,OOO cubic-feet per second, are sufficient to inundate the Delta's floodplain within the levees, sustain riparian corridor vegetation, and stimulate seed germination.5O This flood volume and release rate is on a par with recent flood releases and is likely to occur on average every four years under the present Colorado River management regime unless there is an extended drought.51 c;:; CJ CJ..:i c:..IJ c..:. 1"":: E. Water Dedicated to the Delta Listings are found in Mexico's endangered species act as well, see "Norma Oficial Mexicana que Determina !as Especies Y Subespecies de Flora y Fauna Silvestres Terrestres y Acwiticas en Pe1igro de Extinci6n Amenazades, Raras y !as SujetaB para su Protecci6n:' D.O., 16 de mayo de 1994 (NOM-059-ECOL-I994). 39. See PAYNE ET AL, supra note 23, at 3. 40. Delta habitat is estimated to support 68,000 resident and 49,000 nonresident summer birds. See DANIEL F. LUECKE ET AL, ENVIRONMENTAL DEFENsE FuND, A DELTA ONCE MoRE: REsroRINGRIPARIAN AND WETLANDHABITATlNlHECOlDRAOORIvER DELTA 24 (1999) (citing the calculations found in B.W. Anderson ok Robert D. Ohmart, Vegetation, in INvENToRY AND MoNITORING OFWILDL1FE HABITAT 639 (Allen Y. Cooperrider et al. eds., 1986)). 41. See id. The comparison between river reaches in the United States and Mexico is made to emphasize the importance of the Delta region to the overall lower Colorado River ecosystem. 42. A marsh created and sustained by the irrigation drainage delivered by the Main Outlet Drain Extension canal. See Glenn et al., supra note 11. 43. The water in the MODE is too saline to be included in Mexico's allocation of Colorado River water. The U.S. BOR at one time planned to remove the salt from this water, and the MODE was built as a temporary drain for Wellton-Mohawk agricultural wastewater while the Yuma Desalting Plant was under construction. Completed In 1992, the Yuma Desalting Plant has never been operated due to high costs (estimated to be $25 million annually) and availability of lower-salinity water from other sources. A decision to operate the plant could result in the cessation of flows in the MODE, with devastating consequences for the Cl.enega de Santa Clara. See L.EUCKE ET AL., supra note 40, at 31. 44. See Glenn et at., supra note 11, at 17. A coalition of environmental organizations and research scientists are calling for conservation of the Delta's existing habitat and sufficient 45. See id. at 19. 46. See id. at 16. 47. More field research is needed to quantify with certainty the required volume and frequency of these floods. In addition, freshwater flow needs of Delta fisheries and Gulf near- shore marine species have not been quantified. The flows needed for restoration cited in this article do not include the needs of aquatic species. See generally LUECI<E ET AL, supra note 40, at 17-32. 48. See id. at 42. 49. It is not clear whether the survival of the Delta's riparian vegetation depends on local agricultural return flows or other sources that may recharge the riparian zone during periods in which water does not flow from the United States. See ill. at 20. 50. Fieldwork conducted after the 1997 floods documented high-intensity riparian vegetation in approximately 30 percent of the floodplain,. with evidence of w!despread seed germination of native trees as well as salt cedar. Peak flows of3,500-7,000 cubic feet per second (cfs) inundated nearly the entire floodplain between the levees below Moreloa DlIDI, and diluted significantly the salinity of ocean water in the tidal zone. See id. 51. The 260,000 acre-foot pulse flow represents less than two percent of the Colorado's average annual flow.