Western rivers like the San Juan and the Colorado are mostly fed by burbling mountain streams, which are themselves fed by groundwater and snowmelt.
But a study published on June 5 says those tiny streams are in trouble.
The study in the journal Earth's Future says the base amount of water in streams and rivers that doesn’t come from rain, also known as “base flow,” has been steadily declining since 1950, though that trend varies depending on the specific time period.
“So things like the Colorado, of course, things that are affecting its headwaters are going to have a cascading effect,” says lead author Caelum Mroczek, a research affiliate at Northern Arizona University’s School of Earth and Sustainability. “It's going to have a compounding effect as you go downstream, and so seeing how these things change has a massive impact on regional water supply.”
The majority of base flow comes from groundwater that bubbles up to the surface.
The study’s implications aren’t limited to the past. It predicts that if climate warming continues, by the end of the century, headwater streams could have anywhere from 45% to 65% less base flow to sustain them.
The article comes after the federal government last month released a new Colorado River management plan for the states that share the waterway, and as water levels in Lake Mead reach their lowest point ever. The Colorado River Basin has faced aridification for the last 26 years.
Tiny, ephemeral streams like Flagstaff’s Rio de Flag give mighty rivers the juice to flow freely. But they depend on base flow to do that.
“[The Rio de Flag] could be seen as a headwater stream that leads to the Little Colorado, which leads to the Colorado, et cetera, et cetera,” he explains. “Some of that water then that would otherwise make it to these groundwater systems and things like that that could feed these, you know, the base flow in these larger rivers, might not be making it there.”
Mroczek says less base flow in streams and rivers is a big deal, because that water keeps them alive during dry periods like late summer and times of drought.
By studying 75 years of streamflow data across 11 western states, he says the amount of base flow in streams is shifting earlier in the year, from the summertime to early spring and, now, even late winter.
“The supply is then going to potentially mismatch our demand, and that might mean that we need to figure out storage,” he says. “It's not only that we might see volumetric reductions [in base flow].”
Mroczek’s study used a machine learning model to predict several climate scenarios: One is a “middle of the road” scenario in which the U.S. continues to use fossil fuels at a pretty similar rate as now and doesn’t have massive changes in the production of various fossil fuels. They also modeled a more extreme scenario.
“For what we understand now, if climate were to continue on a very vaguely middle of the road way, then we might expect things to be these reductions of 45% potentially in our annual base flow,” he says. “That is still large, large amounts of acre-feet per year. And those are families that don't have this water, potentially. Now, if you look at a more extreme climate future, you could see greater reductions.”
There’s still time to prevent those scenarios from coming to pass, he adds, but if the study’s predictions are correct, it could translate to way less water in the rivers that families depend on in the arid Southwest.
