KVIG Informative

The Las Vegas Wash was a dry desert channel that ran a few days a year until the valley’s sewage changed that. Seventy years later, the same return flow that once tore the Wash’s banks apart is why Southern Nevada can draw more water from Lake Mead than its tiny legal share would otherwise allow.

The Las Vegas Wash ran only a few days a year before the 1950s, carrying flood water when storms hit the valley and staying dry the rest of the time, according to the Las Vegas Wash Comprehensive Adaptive Management Plan, a joint report maintained by the Southern Nevada Water Authority and the agencies that oversee the channel. Treated wastewater discharged by the City of Las Vegas and the Clark County Sanitation District changed that: the Wash began flowing continuously in 1955, the plan states, calling it a drastic change in the channel’s hydrology.

The U.S. Geological Survey began gauging the Wash’s flow at Pabco Road in 1957, the same year a flood triggered the Wash’s first recorded headcutting, according to the management plan. Through the 1960s, wastewater flows outpaced the growth of the wetlands that had sprung up along the channel, and the excess water began eroding the floodplain. By 1969, channels were visibly cutting through marsh habitat at Three Kids Wash and Northshore Road, the plan states.

Wastewater flow into the Wash rose from about 15 cubic feet per second in 1960 to about 145 cubic feet per second by 1990, as the valley’s population grew, according to the same report. Four major floods, in 1975, 1984, 1998 and 1999, widened the channel and accelerated the erosion; the July 1999 flood alone sent more than 4.5 billion gallons through the Wash. The Northshore Bridge, built in 1978, and culverts elsewhere constricted the flow and sped it up, and trenching for the Las Vegas Lateral pipeline in 1970 and 1971 disturbed the streambed directly.

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Local agencies spent a quarter-century forming committees before they coordinated a sustained response. A citizens’ advisory group organized in 1973 recommended wetland restoration, and the Nevada Legislature created the Clark County Regional Flood Control District in 1985, but a formal erosion task force didn’t follow until 1986, 13 years after the 1973 recommendation, according to the management plan. Clark County voters approved $13.3 million in 1991 for erosion control and a wetlands park, and a dam built under the Northshore Bridge failed within 24 hours of its 1997 construction.

The Nevada Division of Environmental Protection opened the Lake Mead Water Quality Forum in February 1997, bringing local, state and federal agencies to the table monthly, according to the plan. The Southern Nevada Water Authority followed in July 1997 with a 21-member Water Quality Citizens Advisory Committee, which met 18 times over about 10 months, toured the Wash and area treatment plants, and concluded that no single agency could fix the Wash’s problems alone.

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That committee’s recommendations became the Las Vegas Wash Coordination Committee, which the Water Authority organized with 28 members and convened for the first time in October 1998, according to the plan. The timing mattered: three major floods hit the Wash between 1998 and 1999, destroying a temporary grade-control structure that had protected the Las Vegas Lateral since 1984 and forcing the committee’s permanent structures, including one at Pabco Road, into the design phase almost immediately.

The fight to stabilize the Wash was never only about erosion. Every gallon of treated wastewater that reaches Lake Mead through the Wash earns Southern Nevada a return-flow credit against its 300,000-acre-foot Colorado River allocation, the smallest of any state on the river, according to the Southern Nevada Water Authority’s own explanation of how the region’s water supply works. About 40% of the water the authority delivers is used indoors and recycled back through the Wash at a 99% rate, the authority states, which is why the region can withdraw more from Lake Mead than its allocation alone would permit. The committee that formed in 1998 to keep the Wash from washing itself away is still, by extension, the reason Southern Nevada’s water math works at all.