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10 E. p, GLENN ET AL. SaZt Grass Zone We divided the final, intertidal portion of the river into two zones. We designated the west bank of the river as the Salt Grass Zone, since D. paZmeri is the dominant plant on the Baja and Sonoran banks of the river as it approaches the sea and on Montague Island at the mouth of the river. Overall, this zone is only 1.6% vegetated, as the very high tidal amplitude scours the banks of the river and deposits mud over the tide flats. However, the Salt Grass Zone is an important nesting and feeding area for shorebirds (Mellink et aZ., 1996, 1997). CattaiZ Zone The east bank of the intertidal portion of river was designed the Cattail Zone, because it contains Cienega de Santa Clara, the largest Typha marsh in the Sonoran Desert (Glenn et aZ., 1992; Zengel et aZ., 1995). It is maintained by discharge ofagr!cultural waste water from Arizona's Welton-Mohawk Irrigation District via the main outlet drain extension (M.O.D.E.) canal (85% of inflow) and local agricultural drain water (15%) via the Riito canal (Zengel et aZ., 1995). In addition to T. domengensis, it contains seven other common, emergent marsh species (Zengel et aZ., 1995). This unique, 4200-ha wetland supports more than 6000 Yuma Clapper Rails, by far the largest remaining population of this species (Hinojosa-Huerta et aZ., 2001). It also supports the endangered Desert Pupfish (Cyprinodon macularius) (Zengel & Glenn, 1996), plus thousands of migratory and resident waterfowl (Mellink et at., 1996, 1997). It is an important feeding station along the Pacific Flyway. Cienega de Santa Clara appears to be the largest remaining cattail marsh on the lower Colorado River. East of Cienega de Santa Clara along the escarpment that separates the delta from the Gran Desierto, a string of small pozos (springs) bring fresh water onto the salt and mud fiats of the eastern intertidal zone (Glenn et aZ., 1996). These Typha-dominated, pocket wetlands may be part of a long migration route for birds such as the willow flycatcher which travel along the Sonoran coastline to reach the lower Colorado River from wintering areas in southern Mexico and Central America (Garcia-Hernandez et aZ., 2001b). Below the Cienega de Santa Clara, discharge from the marsh system mixes with seawater in an evaporation basin that is only occasionally flushed by high tides. This is important habitat for thousands of shorebirds (Mellink et aZ., 1996, 1997). The Marine Zone The near-cessation of freshwater flow at the river's mouth between 1935 and 1981 had several direct and indirect consequences for the marine portion of the delta. The most obvious result of the decline in freshwater influx has been an increase in the salinity of the water in the estuary and upper Gulf. Early observations (Townsend, 1901) and measurements during controlled releases (Lavin & Sanchez, 1999) indicate that salini- ties in the 32-35% range were quite common. This is in sharp contrast to measurements made since the construction of upstream water diversions. Now, salinities are typically in the 35-45% range (Alvarez Borrego et aZ., 1975; Flessa, pers. obs.). This increase in salinity was most likely the cause of the decline in the population of the bivalve mollusk MuZinia coloradoensis, once the most common species of mollusk in the intertidal zone of the delta (Rodriguez et aZ., 2001b). ,The marine part of the delta is also habitat to two endangered species: the Totoaba (Totoaba macdonaldi) a sciaenid fish, and the Ya~uita (Phocoerlfl sinus), the Gul~ of California harbor porpoise. The Totoaba's dechne IS usually attnbuted to overfishing, bycatch in shrimp nets, and poaching. In addition, increased salinity in the river's ECOLOGY AND CONSERVATION BIOLOGY OF COLORADO RIVER DELTA 11 estuary may have degraded the fish's spawning and nursery grounds (Cisneros Mata CI a~., 1995). The principal source of mortality of the Vaquita seems to be its capture in fishmg nets (Hohn et aZ., 1996; D'Agrosa et aI., 2000), but the role of increased salinity in its key habitat is unknown. , The i.ncr~ase in the salinity of the water in the river's estuary profoundly changed the C1fcul~tlon 10 the upper Gulf of California (Lavin & Sanchez, 1999; Lavin et aZ., 1998; Carb~Jal et aZ., 1997). When the less dense river water entered the estuary, it tended to How mto the Gu~f at the surface, inducing a landward bottom flow of more saline, and I~US d~nser, manne wate!. Such circulation is typical of so-called well-mixed estuaries. Carbajal etal. (1997) estunate that the zone of freshwater mixing extended as far as 60 km from the river's mouth. Their estimate is substantiated by measurements made during con~olled releases (Lavin & Sanchez, 1999) and by isotopic studies of delta shells (Rodnguez et aZ., 2001a). ~ince the diversion o~ much of the river's fresh water, the estuarine circulation is now driven by the evaporation of Gulf water in the river's mouth. High evaporation rates ge~erate d.ense, saline water that sinks and flows along the bottom of the upper Gulf, m ~hile re!at1\~ely le~s dense Gulfwater fl~ws to~ard the estuary near the surface. Today's 'i l:lrculatlon IS typIcal of s~-cal~ed neg~tlve, or Inverse estuaries (Lavin et aZ., 1998). ~ Upstream dams and diverSion projects have also trapped and diverted much of the ~ Colorado's. sediment load. The river once delivered approximately 160 million metric ~ tons of sedunent to the delta every year (van Andel, 1964). Today, that sediment load is ~ almos~ zero and ~aves ~nd the stro~g ~dal c1;UTents are removing the previously ~ ?ep~slted fine:gramed sed~ents (Camqulry & Sanchez, 1999). This sediment rework- ~ 109 I.S ~esponslble for the high turbidity of the upper Gulf's waters. Before the dams, ~ turbidity must have been even higher, but no observations were made t Wayes and tidal currents are capable of removing mud and silt, bu~ coarse-grained , ~atenal such as shells ar~ concen~ated.in beach d~posits known as cheniers (August- l lOu~, 1989). The shell-nch chewers bne the Baja California side of the delta for t a dis~ce of more than 40 km (Kowalewski & Flessa, 1995). The currently active chemers are though to have begun forming after the completion Qf Hoover Dam and the trapping of ~ver sediment in Lake Mead (Thompson, 1968). The cheniers migrate to the west dunng storms and extreme high tides, marking the retreat of the sediment- starved delta. The river not only delivered fresh water and sediment to the marine part of the delta it a~so delivered nutrients. Kowalewski et aZ. (2000) estimate that population densities 'of bivalve mollusks ranged from 25 to 50 specimens per square meter before the dams. In contrast, surveys of current densities show only densities from two to 17 specimens per squar~ meter-a reduction of as. much as 94% from pre-dam values. Other marine orgamsms probably had higher densities as well, as did the waterfowl that fed on them. Kowalewski et at. (2000) attribute the decline in population densities to the lack of river-born nutrients. Indeed, Galindo-Beet et aZ.'s (2000) observation that the size of shrimp cat~es in th~ upper G~ .is positively correlated with the previous year's controlled mflux of nver water mdicates that the river once played a major role in supplying nutrients to the marine life of the delta. Unlike the riparian corridor of the Colorado, the marine portion of the delta has shown little signs of recovery as a result of the delivery of excess flow. It is not yet known what flows might be needed to restore part of the delta's marine life. c:-; C,l CA.' I" !;. ~') 0":> Discussion The most salient feature of the fresh water and brackish flows that sustain the delta is that they are managed flows. They are either agricultural drain waters from the United States and Mexico, or surplus river flows released from United States dams into the