SALT LAKE CITY — An initial analysis of the Colorado River "pulse flow" this spring to Mexico shows some ecosystem success stories, including the spreading of vegetation and groundwater recharge, which will improve the health of the riparian corridor.
The pilot, eight-week release of flows from the Colorado River delivered more than 100,000 acre-feet of water to simulate a spring flood, resulting in the river water reaching the sea for the first time in 50 years.
Hailed as a historic, cooperative event between two countries and bringing together multiple scientific, research and environmental groups, the 53-day pulse flow to Mexico was designed to help the river's parched and saline delta.
Over the course of the experiment and for a couple of months afterward, a team of scientists has been monitoring the impacts of the flow, with its findings announced Wednesday by the International Boundary and Water Commission, the United States and Mexico.
The early analysis shows that less than 1 percent of the total pulse reached the Sea of California and mixed with tidal waters — with surface water first reaching there May 15 — but some of the effects under what's called "Minute 319" were immediate.
Researchers with the monitoring program observed:
The water table rose in response to the pulse flow, with effects decreasing the farther away from active channel.
The waters mimicked a spring flood event and spread the seeds of both native and non-native vegetation, resulting in desired seedlings of trees like willow and cottonwood.
A marked greening of the flood plain.
The pulse flows scoured sediment to a depth of about 3 feet and deposited it to more than twice that depth farther downstream.
The report, prepared by Karl Flessa of the University of Arizona, Eloise Kendy of The Nature Conservancy and Karen Schlatter of the Sonoran Institute, noted that bird populations are expected to change depending on the flow's impacts on vegetation.
While the average abundance of birds along the Colorado River's riparian corridor in the study area has increased from 2002 to 2013, the diversity of species has actually declined.
Numbers of riparian-dependent land birds and breeding water birds are going down, while birds related to agricultural and urban development — such as exotics and raptors — are increasing, according to the authors. The report noted no signficant, immediate change to bird populations, which will nevertheless be observed over time.
Under the binational agreement between the two countries, U.S. and Mexico signed a treaty in 1944 dictating how the waters of the Colorado and Tijuana rivers were to be used. The treaty failed to provide a share of water for ecological protections.
The 2012 agreement that covered the pulse flow detailed how shortages and surpluses would be handled and allows Mexico to store water in U.S. reservoirs. Because of dams and diversions, only rarely has the water from the river reached the sea since 1960 and the delta has diminished to 10 percent of its original size.
A midterm report on the experimental pulse flows is expected to be released in 2016, while a final report is due in 2018.
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