In 2014, a large pulse of water was released into the mostly dry delta of the Colorado River along the U.S.-Mexico border. U.S. Geological Survey scientists are studying the effects of the pulse on the environment as part of a historic, bi-national collaborative effort. The pulse flow and the need to study its effects were accepted as part of the Minute 319 of the 1944 U.S.-Mexico Water Treaty.
A River Ran Through It and Brought Life, At Least for a While
Scientists Study Effects of Water Released Across U.S.-Mexico Border
Results from these studies will be used to assist and inform future bi-national cooperative efforts as the two countries work together to protect resources on both sides of the border.
Research on the effects of the 2014 pulse flow is ongoing, but some results of the flow have recently been uncovered by USGS scientists and their collaborators.
The Colorado River’s Interrupted Flow
The Colorado River starts in the Rocky Mountains of Colorado and, until the construction of Hoover and Glen Canyon Dams, flowed continuously into Mexico and to the Gulf of California. A century ago, the Colorado River Delta was navigable by large boats. Today, upstream diversions and dams in both countries control the Colorado River’s flow, and little to no water is released into the channel downstream of Morelos Dam, which straddles the U.S.-Mexico border.
The now mostly dry Colorado River Delta was once a thriving wetland ecosystem where water and sediment delivered from the Colorado River watershed reached the Gulf of California. Due to the lack of persistent flowing water, much of the Colorado River Delta contains dried out wetlands and degraded riverbanks. The delta provides critical habitat for wildlife and migrating birds, including the endangered southwestern willow flycatcher.
Letting the Colorado River Flow Again to Rejuvenate the Delta
From March 23 to May 18, 2014, 106,000 acre-feet of water were released from the pulse flow, enough to fill about 52,000 Olympic sized swimming pools. An acre-foot is the volume of water required to cover one acre of land to a depth of one foot.
This engineered release of water was the culmination of years of negotiations led by the U.S. and Mexican Sections of the International Boundary and Water Commission in partnership with the U.S. Department of the Interior, and in conjunction with the seven U.S. Colorado River Basin states, Mexican government agencies, and a wide array of municipal agencies, non-governmental organizations and universities from both the U.S. and Mexico.
A Pulse of Life Delivered to the Delta
Did plants grow or become more established as a result of the pulse flow? USGS scientists and collaborators from Mexico and the University of Arizona used satellite data to determine how green and wet the Colorado River Delta became after the flow. Their results indicated that enough water entered the delta to benefit plants. The pulse also recharged groundwater in the delta, an important resource for some native tree and shrub species.
“We observed something very interesting,” said Chris Jarchow, USGS scientist and lead author of the greenness and moisture studies. “In the southern part of the delta, the greenup occurred within the area where the water flowed overland, or the inundation zone. However, in the more northern portion of the study area the greenup occurred well beyond the zone of inundation. These findings support groundwater-level measurements that showed the aquifer rose in response to the pulse, allowing vegetation to take advantage of the shallower water table.”
The Colorado River Delta was greener after the 2014 pulse flow compared to 2013 pre-flow conditions, indicating the pulse flow increased the amount of vegetation in the delta. Although greenness declined in 2015, it was still greener in 2015 compared to 2013, indicating that the single pulse flow in 2014 stimulated plant growth for at least one year.
Sediment in the Pulse
Other USGS scientists and collaborators from Mexico and Utah State University evaluated how the pulse influenced the physical features of the delta or the transport of soil particles in water (sediment). Understanding sediment movement is important because the combination of flowing water and sediment can bury plants and alter the path of river channels, creating sandbars. Newly created sandbars can serve as habitat for plants and aquatic species.
Study results showed the pulse flow was too small to generate large channel changes. In fact, the pulse flow, although historic, was only five percent of the magnitude and 10-15 percent as long in duration as seasonal peak flows typical during the early 20th century. Scientists only found small changes (at the scale of a few feet) to the riverbed with some sediment removed and redeposited short distances away.
“Following the completion of upstream dams in the mid 20th century, the river channel in the delta was completely transformed, and the drastic reduction in water and sediment supply caused the waterway to narrow and become deeper,” said Erich Mueller, USGS scientist and lead author of the sediment study. “As a consequence, sediment erosion and deposition during the pulse flow was restricted to the relatively narrow channel with deeper depths and faster flows.”
Optimism for the Colorado River Delta?
Although the 2014 pulse flow was too small to generate changes in the physical features of the Colorado River Delta, it did rejuvenate the vegetation in the delta for at least a year. This rejuvenation is important because native trees, like Goodding’s willow and Fremont cottonwood, rely on floods for seed germination. Additionally, a recent study by the University of Arizona and USGS suggests that bird species in the Colorado River delta appear to prefer native trees over the nonnative tamarisk (also known as salt cedar) when foraging, and the enhanced vegetation that resulted from the pulse flow has expanded the feeding habitat birds are utilizing.
It is not clear how long the effect of a single pulse flow lasts or what the long-term effects of multiple pulse flows will have on the Colorado River Delta. Ongoing studies are being conducted by the USGS, as well as other federal and state agencies, Mexican collaborators and universities to determine the full effects of the pulse flow.