Francisco Zamora-Arroyo’s research while affiliated with Sonoran Institute and other places

The Colorado River Delta (CRD) consists of agricultural fields and remnant wetlands that are sustained by the Colorado River and agricultural drainage. Electrical conductivity (EC), dissolved oxygen, and ammonia‐nitrogen data were collected at different locations in the CRD at the end of a wet period in 2000 and compared with the concentrations present during the dry conditions that prevailed from 2001 to 2020. An increase in mean EC values measured by the International Boundary and Water Commission at Morelos Dam between wet (1.2 ± 0.06 mS/cm) and dry (1.4 ± 0.11 mS/cm) periods was observed. The same was also observed at reach 4 of the riparian corridor between the wet (2.4 ± 0.24 mS/cm) and dry (3.3 ± 0.75 mS/cm) periods with an increasing trend in salinity over the years. However, salinity values did not exceed the limits required for the growth of native trees and cattails. A positive result was that EC values at reach 6 and the Hardy River significantly decreased over the past 13 years, this was attributed to inflows from “Arenitas” Water Treatment Plant and its wetland that began operations in 2008. Finally, although severe drought will likely mean water shortages, results indicate that treated water can maintain and restore riparian corridors and wetlands. It is recommended that ongoing conservation efforts consider these local water sources in addition to the ecological pulse flows.

We are among the scientists who have documented the environmental and ecological changes to the Upper Gulf of California following the reduction in the Colorado River’s flow. We object to any suggestion that our research supports Manjarrez-Bringas et al.’s conclusion that the decline in the Colorado River’s flow is the reason for the decline in the population of the endangered vaquita porpoise (Phocoena sinus). Manjarrez-Bringas et al.’s conclusions are incongruent with their own data, their logic is untenable, their analyses fail to consider current illegal fishing practices, and their recommendations are unjustified and misdirected. Vaquita face extinction because of bycatch, not because of the lack of river flow.

Drastic alterations to river hydrology, land use change, and the spread of the nonnative shrub, tamarisk (Tamarix spp.), have led to the degradation of riparian habitat in the Colorado River Delta in Mexico. Delivery of environmental flows to promote native cottonwood (Populus spp.) and willow (Salix spp.) recruitment in human-impacted riparian systems can be unsuccessful due to flow-magnitude constraints and altered abiotic–biotic feedbacks. In 2014, an experimental pulse flow of water was delivered to the Colorado River in Mexico as part of the U.S.-Mexico binational agreement, Minute 319. We conducted a field experiment to assess the effects of vegetation removal, seed augmentation, and environmental flows, separately and in combination, on germination and first-year seedling establishment of cottonwood, willow, and tamarisk at five replicate sites along 5 river km. The relatively low-magnitude flow deliveries did not substantively restore natural fluvial processes of erosion, sediment deposition, and vegetation scour, but did provide wetted surface soils, shallow groundwater, and low soil salinity. Cottonwood and willow only established in wetted, cleared treatments, and establishment was variable in these treatments due to variable site conditions and inundation duration and timing. Wetted soils, bare surface availability, soil salinity, and seed availability were significant factors contributing to successful cottonwood and willow germination, while soil salinity and texture affected seedling persistence over the growing season. Tamarisk germinated and persisted in a wider range of environmental conditions than cottonwood and willow, including in un-cleared treatment areas. Our results suggest that site management can increase cottonwood and willow recruitment success from low-magnitude environmental flow events, an approach that can be applied in other portions of the Delta and to other human-impacted riparian systems across the world with similar ecological limitations.

As provided in Minute 319 of the U.S.-Mexico Water Treaty of 1944, a pulse flow of approximately 132 million cubic meters (mcm) was released to the riparian corridor of the Colorado River Delta over an eight-week period that began March 23, 2014 and ended May 18, 2014. Peak flows were released in the early part of the pulse to simulate a spring flood, with approximately 101.7 mcm released at Morelos Dam on the U.S.-Mexico border. The remainder of the pulse flow water was released to the riparian corridor via Mexicali Valley irrigation spillway canals, with 20.9 mcm released at Km 27 Spillway (41 km below Morelos Dam) and 9.3 mcm released at Km 18 Spillway (78 km below Morelos Dam). We used sequential satellite images, overflights, ground observations, water discharge measurements, and automated temperature, river stage and water quality loggers to document and describe the progression of pulse flow water through the study area. The rate of advance of the wetted front was slowed by infiltration and high channel roughness as the pulse flow crossed more than 40 km of dry channel which was disconnected from underlying groundwater and partially overgrown with salt cedar. High lag time and significant attenuation of flow resulted in a changing hydrograph as the pulse flow progressed to the downstream delivery points; two peak flows occurred in some lower reaches. The pulse flow advanced more than 120 km downstream from Morelos Dam to reach the Colorado River estuary at the northern end of the Gulf of California.

River regulation in semiarid lands often results in a dry riverbed, which conducts to serious ecological degradation. Given the numerous hydraulic control structures, in the U.S. area, and the water diversions for consumptive uses, in Mexican territory, nowadays the Colorado River Delta region shows runoff only during wetting years when surplus water is available. Due to concerns related to the ecological health of this Delta, an international agreement (Minute 319) was signed between Mexico and the U.S. in November 2012. This agreement contemplates, among other issues, the release of water for environmental restoration purposes in the Colorado River Delta riparian corridor. The purpose of this study is to propose feasible discharge flows, timing, and extension of flooded areas for riparian restoration. This is accomplished through the analysis of sediments texture, soil salinity, and groundwater levels, which were conducted in conjunction with diverse base and pulse flow modeling scenarios in a 10-mile reach of the Colorado River. Infiltration from irrigation channels, irrigation returns, and river discharge flows was recognized and the depth to water table and its influence on riparian vegetation was analyzed. Under a pulse flow of 200m3s-1, the resulting flood will cover 65% of the entire study area, similarly to the inundation observed in 1997, the year of the last natural flood. Modeling results further suggest that, the mainstream of the river will be flooded by mimic base flows of 10m3s-1. In addition, the timing and elapsed time of discharges that mimic base and pulse flows could play an important role to maintain shallow groundwater levels and to promote native riparian vegetation establishment.

The Cienega de Santa Clara is a brackish coastal wetland located in the Colorado River Delta Biosphere Reserve. It plays vital ecological roles that support endangered-migratory species and flora. Its main water supply comes from irrigation return flows from the United States with salinity levels (2–3 g L−1) that promote the growth of Typha domingensis. This study conducted a water-mass balance during one year of reduced water volumes and high salinity levels caused by a trial run of the Yuma Desalting Plant. The analysis projected the effects that could be expected from the plant with permanent operation conditions. During the test run, inflows varied from 0.62 to 5 m3s−1, supplying the wetland with 127.20 hm3 of water at 2–5 g L−1. Forty-five percent of this volume exited as runoff, while 52 % exited by evapotranspiration. Water depth varied from 0.10–2.1 m at different time responses; also, during the summer water salinity levels increased above the tolerance limit for the vegetation, reducing its area by 20 %. Vegetation area returned when normal flows were resumed in the fall. The results indicate that permanent operation of the plant would reduce the vegetated area in the wetland due to reduced flows and increased salinities.

A large pulse of water is planned to be released into the dry Colorado River channel in Mexico. This engineered experimental spring flood, which will flow from Lake Mead and pass through downstream reservoirs, is the culmination of decades of applied research. The pulse flow is a rare opportunity for research at the landscape scale [Glenn et al., 2013].

An obstructive tidal sandbar forms in the estuary of the Colorado River, Mexico, during periods of low fluvial discharge. The sandbar typically develops at a bedload convergence zone centered approximately 30 km landward from Montague Island near the river's mouth. The estuary provides important spawning habitat for the endangered totoaba (Totoaba macdonaldi) and commercially exploited Gulf corvina (Cynoscion othonopterus), as well as habitat for larval shrimp, all of which are impacted by reduced freshwater/sea water mixing when the sandbar is present.Sequential satellite images, aerial photographs, overflights, and ground observations were used to document the geomorphology of the tidal sandbar and its formation, removal, and subsequent reappearance in response to long-term variations in fluvial flow following completion of Glen Canyon Dam in 1964. Evidence for the formation of the sandbar during low-discharge periods prior to anthropogenic manipulation of the river's flow was also examined.Water data sensors were installed upstream from the current sandbar to monitor the frequency of tidal overflow and document the effect of tides on salinity. Topographic surveys of the sandbar were completed both before and after the 2010 Mw 7.2 El Mayor-Cucapah Earthquake which caused subsidence of lands immediately east of the sandbar's crest.Utilizing topographic data and knowledge of the sandbar's geomorphologic and sedimentary history, we designed and installed a system of dredged pilot channels to improve connectivity between the upper and lower estuary by facilitating tidal and fluvial flow across the barrier. The effectiveness of the pilot channels is currently under evaluation. If the pilot channels fail to establish the desired level of connectivity, installation and maintenance of a more substantial dredged channel may be necessary to meet restoration objectives.

Evapotranspiration (ET) and other water balance components were estimated for Cienega de Santa Clara, an anthropogenic brackish wetland in the delta of the Colorado River in Mexico. The marsh is in the Biosphere Reserve of the Upper Gulf of California and Delta of the Colorado River, and supports a high abundance and diversity of wildlife. Over 95% of its water supply originates as agricultural drain water from the USA, sent for disposal in Mexico. This study was conducted from 2009 to 2011, before, during and after a trial run of the Yuma Desalting Plant in the USA, which will divert water from the wetland and replace it with brine from the desalting operation. The goal was to estimate the main components in the water budget to be used in creating management scenarios for this marsh. We used a remote sensing algorithm to estimate ET from meteorological data and Enhanced Vegetation Index values from the Moderate Resolution Imaging Spectrometer (MODIS) sensors on the Terra satellite. ET estimates from the MODIS method were then compared to results from a mass balance of water and salt inflows and outflows over the study period. By both methods, mean annual ET estimates ranged from 2.6 to 3.0 mm d(-1), or 50 to 60% of reference ET (ETo). Water entered at a mean salinity of 2.6 g L-1 TDS and mean salinity in the wetland was 3.73 g L-1 TDS over the 33 month study period. Over an annual cycle, 54% of inflows supported ET while the rest exited the marsh as outflows; however, in winter when ET was low, up to 90% of the inflows exited the marsh. An analysis of ET estimates over the years 2000-2011 showed that annual ET was proportional to the volume of inflows, but was also markedly stimulated by fires. Spring fires in 2006 and 2011 burned off accumulated thatch, resulting in vigorous growth of new leaves and a 30% increase in peak summer ET compared to non-fire years. Following fires, peak summer ET estimates were equal to ETo, while in non-fire years peak ET was equal to only one-half to two-thirds of ETo. Over annual cycles, estimated ET was always lower than ETo, because T. domingensis is dormant in winter and shades the water surface, reducing direct evaporation. Thus, ET of a Typha marsh is likely to be less than an open water surface under most conditions.