As I wade into the east fork of Montana’s Bitterroot River with my fly rod on an August afternoon, I count myself lucky to feel a swift current. The water is at a normal level for late summer, and that is becoming a rare treat in the Central Rocky Mountains, a region that extends from the Canadian border through Wyoming. Since 1951, average late-summer stream flows have been dropping. And while that’s troubling news for trout fishermen, it could be disastrous news for native trout.
With several other scientists, I began studying Central Rockies stream flow four years ago after several hot, dry summers reduced flows to the point of endangering aquatic species and forced the closure of popular blue-ribbon rivers. Declining flows raised concerns about the already threatened bull trout, and elevated water temperatures in Montana’s Big Hole River posed an immediate threat to the last arctic grayling population in the Lower 48.
Was this just a temporary anomaly, or was there a long-term trend at work? Was this caused by climate change? The fly-fisherman in me worried about my beloved trout. The scientist in me thought back to the Intergovernmental Panel on Climate Change prediction that, as a result of global warming, the Central Rockies will start to experience lower river levels in summer much more often.
Like many others, I wanted answers, so I joined colleagues at the University of Montana to study long-term stream flow trends in more than 150 Central Rockies streams using U.S. Geological Survey records from 1951 to 2008. What we found was alarming: Our analyses indicate that 89 percent of non-regulated watersheds – undammed rivers – are experiencing significant declines in stream discharge.
In simpler terms, late-summer stream flow has been declining since 1951, often causing visibly lower August stream flows in many rivers across the West. Although limited historical data make it difficult to determine the direct cause of this trend, various pieces of the puzzle point toward rising average temperatures across the globe – climate change – as the culprit. In the Central Rockies, long-term warming has caused a multitude of changes to the hydrologic cycle. Researchers have documented decreases in mountain snowpack, more rain falling during winter, earlier spring runoff in rivers, and increased evaporation during hotter summers.
To understand the potential causes of these trends, we selected a number of USGS gauging stations at pristine sites – undammed streams with no water diversion – and then examined the relationship between stream flow and a number of other variables (like average air temperatures). Not only did we find that air temperatures were substantially warmer now than they used to be, we also found a strong relationship between these warmer air temperatures and declining stream flows at most of the pristine sites.
Regardless of the cause, lower stream flows could have severe consequences for trout. Less water in streams during summer means less habitat and potentially warmer water temperatures.
As I work upstream, carefully casting my fly behind each boulder, questions fill my head. What does the future hold for native trout in the Central Rockies? Will they adapt? Will they disappear? What can we do? With scientists projecting continued warming in the region, the future looks ominous for native trout.
There are many aspects of this region’s future that are uncertain, but one thing has become clear: Species that depend on cool water, such as trout, will be under a lot more stress in the years to come.
For a more details on the research, see the September online issue of the Journal of Climatic Change. DOI: 10.1007/s10584-011-0235-1.