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50 F. ZAMORA-ARROYO ET AL their riparian corridors (Dynesius & Nilsson, 1994; Nilsson et al., 1997). In south- western United States, flow-regulated riparian zones have been damaged by loss of the natural pulse-flood regime, that formerly washed excess salts from riverbanks and germinated native trees (Briggs, 1996; Busch & Smith, 1995; Poff et aI" 1997; Stromberg, 1998a). On the lower Colorado River, the largest and most altered river in the south-western United States and Northern Mexico, an exotic, salt tolerant shrub, Tamarix ramosissima (salt cedar), in association with native halophytes, has almost completely replaced the mesophytic native forest that historic\\lly dominated the riparian corridor from Grand Canyon to the delta on the Gulf of California (Ohmart et aI" 1988; Busch & Smith, 1995). Loss of Populus fremontii (Fremont's cottonwood) and Salix gooddingii(Goodding's willow) trees and associated epiphytes and understory plants, has lead to a collapse of supporting habitat for numerous species of plants, birds, mammals and reptiles, so that today 45 species in the lower Colorado River ecoregion are listed as sensitive, threatened or endangered (U.S. Bureau of Reclamation, 1996). Deterioration of native habitat on regulated rivers cail be progressive and irreversible (Nilsson et al., 1997). It is not known if restoring elements of a natural flow regime would by itself permit native species to repopulate south-western riparian zones (Briggs, 1996). We report on the effects of pulse floods on vegetation in the delta region of the Colorado River, below the last diversion of water at Morelos Dam in Mexico. These discharges began with the filling of the last large reservoir on the river, Lake Powell behind Glen Canyon Dam, in 1981 (Glenn et aI., 1996, 1999). The entire discharge of the Colorado River is considered to be over-apportioned for human use with no water remaining for ecosystem maintenance (Morrison et aI., 1996). However, the flows of arid-zone rivers are inherently variable. The river's dams are operated to keep the large reservoirs full, to accommodate electric power generation, recreation, and storage for downstream water use. Hence, when there is excess runoff in the watershed, large volumes of so-called 'waste spills' are released to the Colorado River delta and upper Gulf of California. Since 1981, the major releases have been associated with ENSO cycles in 1983, 1993 and 1997, which brought above- normal winter or spring precipitation into the lake system (Glantz et aI., 1996; Li & Kafatos, 2000)(Fig. 1). We analysed the effects of river discharge on abundance of vegetation from 1992 to 1999 and characterized the species composition of existing vegetation using remote sensing and ground methods. We found that pulse floods have reestablished cohorts of native trees in a 100 km stretch of the riparian zone of the delta, and that the extent of vegetation cover in this stretch is responsive to flood flows. Materials and methods Study area andjlow data The general study area is the zone of natural (non-agricultural) vegetation in the riparian corridor from Morelos Dam to the mouth of the Colorado River in the Gulf of California (Fig. 2). The main vegetation types and hydrological features of this zone have been described and entered into a Geographic Information System database (Valdes-Casillas, 1998), The riparian corridor is confined within 6 m tall, earthen levees that keep flood waters out of adjacent agricultural fields. The corridor is < 2 km wide in the northern stretch as it passes through the agricultural district, then widens to 30 km in the southern stretch as it approaches the ocean. The river in the delta is a series of braided channels interspersed with straight sections which have been dug to facilitate water movement, Our primary interest for the present report is the 100 km (14,316 ha) stretch of river from Morelos Dam to the junction of the Colorado River with the Hardy River which contains a mixture of native trees and shrub vegetation. Below the juncture the river I t i r i ~ I ~ ~ ~, ~ p I ~ ~ I~ ~ 'if * ~ tt f;:' ~: % ~: ~ ~ (i t f' ~ ~ ~ ~ ~ ~ ~~ ~ REGENERATION OF TREES IN RESPONSE TO FLOOD RELEASES 2??oo 51 o 1970 1990 2000 1980 Year . Figure 1. Annual water releases from the United S . , the Southerly International Boundary Th's ,tates .to ~exlco In the Colorado River below water and flows to the Gulf of Califo~ia ~:;~g~n~ staCtJon IS bel~w the last ?ive;-;ion point for g e olorado River delta npanan zone. 1960 carries saline agricultural drain water to th . native trees. . e sea and IS apparently too saline to support We used a combination of low-level aerial surve wens to document the vegetation es and ys, ground .~sects and monitoring locations are shown in Fig 2) WetythP hI yddrology of this nver stretch (sampling . .. en corre ate past eta' ' events, using satellite images of the delta and fl dV ge no':1 cover With past flow Boundary and Water Commission US De t ow ata prOVided by International measured at the Southerly Internati~n~1 130 ~. of State, EI Paso, Texas, USA. Flows for water, were assumed to flow to the sea u~ ary' 3.~ km b~low ~e last diversion point (AI Goeff, IBWC, pers. comm.). WI a resl ence time In the delta of 3-5 days ~ .~ i f. I, ~ t k " ~ ~ & ~ f 1\. I I Aerial SUI'VeyS We conducted a medium-altitude (lOOOm) 'al documented by oblique videography to ga' aen su~ey on 27 February 1997, . . ' In an overview of geomo hId vegetation of the nparian ecosystem and to rp 0 ogy an during a release event of known m . obse~e patterns of water distribution (150 m) aerial photographic survey o~~~~' This was foll~wed by a low-level releases, using a multiband (red blue and NIR) ad~ ,19199, follOWing 3 years of water 2001). ' Iglta camera (DyCam) (Nagler et aI., DyCam images taken within the nati ( % of bare soil, trees, shrubs and groun;:o~:~ ~~n~ n = 63) were used to. determine the approximately 67 x 100 m ground area Each h e present s~dY. Eac~ Image covered viewing program and overlain with a iliin-li~d O~Oograo ph,was l~dPorted Into a computer , -pOint gn . Land cover class was