Is the Forest Service doing all it can for the Colorado River?

George Sibley

I am grappling with a few obstreperous facts, the first two from the Western Water Assessment 2020 study of Colorado River “Climate and Hydrology Science”:

Historically, approximately 170 million acre-feet (maf) of precipitation falls on the Colorado River Basin on average annually — but less than 10% of that makes it into the surface flows of the Colorado River system, now averaging around 13-14 maf annually. (A million acre-feet is 326 billion gallons of water, enough for roughly two million households.)

Around 85% of the precipitation for the entire Colorado River Basin falls on the mountains above 8,000 feet, most of it as winter snowpack. 

The whole Colorado River Region — the natural basin plus all out-of-basin destinations for the river’s water — is severely stressed by growing water shortages that are affecting all water-related activities. That includes 4 million acres of agriculture important to the national food supply, hydropower production, industries (including a fast-growing outdoor recreation industry), not to mention some or all of the domestic water for upwards of 40 million people.

Most of the headwaters where the Colorado River’s water begins is forested land, under the management of the U.S. Forest Service, whose original administrative charge was to manage the National Forests “to improve and protect the forest, or to secure favorable conditions of water flows, and to furnish a continuous supply of timber.” 

A big question that emerges out of the first fact is: What happens to all that annual precipitation that falls over the Colorado Basin — most of it over our mountains as winter snow?

We know a few things about that. We know that surface water exposed to the sun evaporates, changes to water vapor under the influence of the sun or any heat source. Evaporation is less at higher elevations, but still a force there eroding the surface water supply. 

We also know that maybe as much as a third of the snow that falls on the headwaters region never even becomes liquid water; it is sublimated — changed from the solid state directly to the vapor state — by sun and wind. We can do little about that in the mostly treeless rock-and-ice alpine zone, where the wind always blows and the sun’s short-wave energy is fierce, even in the winter. 

But significant sublimation also happens in the subalpine and montane forests below timberline. Snow that gets all the way down to the ground between trees is safely shaded and sheltered from the sun and wind. However, a lot of snow is intercepted by tree branches, especially in dense stands, and the sun, heating the dark-colored branches, sublimates the snow caught on the branches. Huge quantities of snow thus vaporize through the winter without ever turning to liquid water.

We also know enough about how plants work to know that they consume large quantities of the water that soaks into the soil of the root zone, and transpire most of that into the atmosphere as vapor to cool their micro-environment (the way we sweat to cool ourselves). A mature spruce tree might transpire 100 gallons a day; a lodgepole pine, 40-50 gallons; a riparian cottonwood, 500+ gallons. 

We know that hotter, drier weather increases all those forms of vaporization.

In short, these factors — sublimation, transpiration and evaporation — eliminate a huge quantity of the precipitation that falls on the headwaters and its forests before it even gets to the places where ecosystems containing humans depend on the water. This will increase as the climate warms. An additional 4-6% of the surface water supply will vaporize with every additional degree of climatic warming.

To cut right to the chase here: In the headwaters of the most stressed river in the most arid part of the nation, could or should the Forest Service be managing our National Forests — the source of most of the river’s water — with a high priority to optimize “favorable conditions of water flows?”

I hasten to add that optimizing water flows is not the same as maximizing water flows.  The amount of water flowing from mountains is easily maximized by removing all the trees and other vegetation, but the resulting flows also remove soil and everything else in their way. That “maximization of flows” is already happening too frequently due to wildfire.

But there are creative management opportunities that could make a difference in the optimal quantity and quality of water that makes it through the gauntlet of high-altitude sun, wind and forests. For example, the standard silvicultural practice in replanting burns or log-overs, and doing the noncommercial and commercial thinnings in regenerated stands, is to leave trees close to each other because then they grow tall and straight with minimal “branchiness,” which makes them good sawtimber — that being the traditional management priority.

But if your management priority were to optimize water production, less dense stands would have less sublimation from interception on branches and more snow would get to the ground as snowpack. Fewer trees would also mean less transpiration. You do need enough trees to shade and shelter the snowpack, and hold the ground against undue erosion. But there is a balance there that needs to be formally explored as even the headwaters dry out.

Reading through the draft Forest Plan proposed for the Grand Mesa Uncompahgre Gunnison (GMUG) National Forests through the 2030s, however, I find no recognition of the larger regional urgency. The Colorado River is never even mentioned by name, even though the plan is for much of that river’s headwaters, where 85% of the water originates for 40 million people and four million acres of productive agriculture beyond the forests. 

The plan has a segment on “improving and restoring” 15% of the 48 GMUG watersheds over its expected 15-year life span, and it acknowledges some local impacts of climate change, but I find no acknowledgement of the forests’ larger responsibilities to the entire Colorado River region, which truly needs a priority Forest Service commitment to optimizing favorable water flows out of the GMUG forests in an increasingly desperate time of diminishing water supply.

(George Sibley is a Gunnison writer and former director of the Upper Gunnison River Water Conservancy District.)