A dry year on dry year tests spruce, aspen and trout alike

The water year closes in two weeks, and the tally for the headwaters above these forests is close to set. The three snow-survey stations that measure what falls on the high country here, Burro Mountain 11 kilometers away at 9,290 feet, Bison Lake 24 kilometers out at 10,950 feet, and Trapper Lake 42 kilometers out at 9,750 feet, have taken in 78, 79 and 70 percent of their median precipitation for the year so far.¹ All three are bare of snow, and the accumulation season at these sites does not begin until late October.¹ Barring a wet last half of September, those numbers stand.

In Garfield County, this same week last year, extreme drought covered the whole county and none of it was in the exceptional category, the most severe one on the map.² Exceptional drought stayed absent through the winter, appeared across a quarter of the county in the last week of March, spread to three-quarters by late April, and held near half the county from late May through August.² As of the week of September 8 it covers 34 percent of the county.² Ninety-seven percent remains in extreme drought or worse, as it has for nine weeks running.²

A dry year following a dry year matters more to the trees here than either year alone. Engelmann spruce fight off spruce beetle with resin, and a tree's ability to produce it depends on water. Tree-ring reconstructions of past outbreaks in northwestern Colorado spruce forests, published in the journal Ecology by Hart, Veblen and colleagues, found that outbreak timing tracked drought more closely than it tracked temperature. Experiments on Engelmann spruce found photosynthesis falling off once stem water potential dropped past a threshold under both short and chronic water stress. A spruce that spent last summer drawing down its reserves enters this one with less to spend. Aspen fail differently: severe drought cavitates the vessels that carry water from root to leaf, and the stands most exposed are the low, south-facing ones. Weakened aspen then draw poplar borers, aspen bark beetle and Cytospora canker.

The fire season followed the same curve. The Dry Creek Fire burned 315 acres south of Rifle in late June. The Elk Knob Fire, a lightning holdover in the Flat Tops Wilderness, was contained August 19. The Derby Fire near the Flat Tops prompted evacuation notices around August 20. By mid-September the whole of the White River National Forest was under Stage 1 fire restrictions, which bar campfires anywhere outside developed sites, with the Forest Supervisor noting that conditions remain dry, particularly at the higher elevations.³ Satellite fire-detection systems, which flag heat signatures from active burns, have shown no such detections within 50 miles in the past five days.⁴

In the creeks, the USGS streamgauge on Elk Creek at New Castle read 1.33 cubic feet per second this morning, down from 16.5 a month ago.⁵ That gauge sits at the valley mouth around 6,800 feet, below irrigation headgates that stay open on the Western Slope through October; it reads what is left of the creek after diversions, not the headwater flow itself. Brook trout in the Flat Tops drainages spawn once water drops below 9 degrees Celsius in October, and their eggs need groundwater moving up through clean gravel to stay oxygenated. Low fall water can cut fish off from those sites and starve the gravel of flow. Native cutthroat trout, logged on iNaturalist last October along Mitchell Creek near Glenwood Springs, run a narrower thermal window, with mortality setting in above roughly 19.6 degrees Celsius.⁶

The Middle Colorado Watershed Council, based in this stretch of the Colorado River valley, tracks streamflow, forest condition and fire risk across this drainage.