The spring of 2011 was a particularly high runoff year in Colorado’s northern mountains. In response
to the huge snowpack, Dillon Reservoir, which is part of the Denver Water system, was drawn down significantly prior to the spring snowmelt to reduce the flood risk for the town of Silverthorne, located below the dam. Bob Steger, Manager of Raw Water Supply for Denver Water, described it as “a very challenging balancing act.” After filling began, the reservoir level came close to the spillway crest, but forecasts indicated
that the inflows would soon begin going down. Unfortunately, the inflows increased and stayed high for nearly a week longer than the forecast, raising concerns that significant flooding would occur. Luckily, inflows started going down just in time to avoid a serious flooding event. This incident and others like it led some to suspect that widespread tree death due to bark beetle infestations might
be responsible; reducing forest uptake of water and increasing the amount of water in surface streams would have major consequences for water resources management in snow-dominated mountain regions.
The hydrologic impacts of such a large landscape changes can last for decades, giving greater impetus to understanding how these infestations affect surface water supplies.
There was reason to believe that was occurring–but as it turns out, the story of how tree death from beetle infestations affects water supplies is far more complicated than simply turning off the “straws” of water uptake by trees.
Before we discuss that story in more detail, though, what are these tiny insects that have killed so many trees in western North America and are worrying water managers? The term “bark beetles” refers to a family of insect species, all of which eat through bark and the living tissue of trees. In the West, the most abundant of these bugs is the native mountain pine beetle, which feeds off a variety of trees such as lodgepole pine, ponderosa pine, and limber pine. Normally, bark beetles kill relatively few trees each year, but under certain conditions they are able to multiply rapidly and spread across entire landscapes. Scientists believe that the current beetle outbreak, which began in the mid-1990s, was spurred on by the drought that has affected much of the west since the early 2000s.