Early Snowmelt Could Mean Disaster for River Ecosystems

Hidden in the forests of northern Arizona, mountain ecosystems teeter on the brink of devastating change.

These habitats in the rivers and streams that spill over the upper elevations of the state are home to plants and animals that depend on a threatened supply of water—a resource that is directly influenced by the rising temperatures and earlier spring snow melt that scientists have recorded.

Apache trout being released in Arizona stream.
Credit: U.S. Fish and Wildlife Service

“For humans it may be an inconvenience for rations of water, whereas for the ecosystem it is a matter of life or death,” said Iris Stewart, an assistant professor at the Environmental Studies Institute at Santa Clara University.

Stewart is the lead investigator of a group that has been studying the timing of stream flows in the western United States and Canada. Using a system of about 300 gages that measure the amount of water in snowmelt-dominated waterways, she has been able to track the time at which the majority of melting snow runs off mountains into streams and rivers.

Stewart and her colleagues concluded that a sizable fraction of their findings were consistent with observed changes in global and regional temperatures.

• Widespread warming (1.8-5.4 degrees over the past 50 years)
• Significant decreases in snow packs (the accumulation of winter snow fall on mountain regions)
• Advances of one to four weeks in timing of spring snowmelt runoff

The area found to be most affected was the Pacific Northwest, while several gages in New Mexico proved to be contrary to the trend. However, Stewart predicts that by the end of the 21st century, the time at which the majority of water is shed into snow-fed streams will be 30 to 40 days earlier. If things continue in a business-as-usual scenario, the impacts may be seen with greater severity in the Southwest’s mountain watersheds.

Over the years, the southwestern states have found ways to secure a water source for their residents, whether by regulating rivers with diversions and dams or by digging deep into the ground. Although not all these methods are completely accepted as flawless cures to the region’s water woes, they have made it possible for the arid Southwest to sprout major metropolitan areas amid a desert.

The threatened ecosystems of Arizona’s fragile river and stream bank areas are not so insulated. If spring water flow arrives 40 days earlier and the monsoon season continues to begin in mid-June or early July, fish and every other organism that relies on snowmelt-fed streams will find themselves exposed to an even longer dry period.

The result is that many species may not survive, said Shaula Hedwall, a biologist with the U.S. Fish and Wildlife Service.

“I expect that it will even further limit the amount of habitat,” she said. Hedwall and her colleagues have had to undertake intensive management efforts to maintain populations of some aquatic species.

Hedwall specializes in a number of fish species, including the Little Colorado spinedace (Lepidomeda vittata), a minnow native to the Little Colorado River watershed. This silvery fish rarely grows larger than four inches and is currently listed as threatened due to loss of habitat, use of fish poisons, and the introduction of non-native competitive species.

Stewart’s findings could spell disaster for what remains of the spinedace, said Hedwall. It is a concern that is representative of what is threatening ecosystems across the West.

Many fish species instinctually spawn during the spring runoff when water temperatures begin to rise. An earlier streamflow and increased water temperature may trick them into spawning earlier, but biologists expect the fish will be able to adapt to a change in spawn time.

“The issue is survival through the year, not spawning,” Hedwall said. Even if the fish spawn following a high-flow event, with longer, drier summers, it is likely that the young fish won’t have enough habitat to survive.

An extension of the dry period before the monsoon, coupled with weakened streamflows due to decreases in snow packs, will cause an even further loss of riparian habitat. Biologists have already seen the disappearance of many clear-flowing streams and their replacement by disconnected pools for a majority of the year, said Hedwall.

Fish populations are relative to the size of their habitat, and Hedwall has noticed a decrease in the number of spinedace over the last decade. When the streams run dry, survivors are confined to severed pools with limited space and resources. Although the current drought may be part of the natural process, some studies have shown that the effects of climate change could exacerbate its severity in ecosystems.

Other fishes aren’t faring so well either.

“Apache [trout] in small streams are really crashing,” said Mike Lopez, a biologist with the Arizona Game and Fish Department. He specializes in the Apache trout (Oncorhynchus apache)—another fish that is clinging to what is left of its habitat.

The state fish of Arizona, the Apache trout is one of only two trout species native to the state. As a threatened species that has been making a slow comeback, it is vulnerable to changes in its White Mountain environment in northeastern Arizona and has already been the subject of extensive recovery efforts.

Like the spinedace, the Apache trout will probably be able to adapt to an earlier spawn time, said Lopez. However, the fish that live in small streams may see a reduction in the number of hatching eggs, and those that survive may not make it to adulthood.

Those that do could find themselves fighting a losing battle for shrinking real estate against competitive and occasionally predatory non-native species such as the rainbow and brown trout.

In either event, Lopez and Hedwall agree that sustaining populations of threatened fish in the world that Stewart predicts will require a great deal of human intervention.