Streamflow

The seasonal timing, magnitude, and duration of high and low streamflows can affect the growth and accumulation of benthic algae (fig. 4). In the upper White River Basin, peak streamflow is driven by snowmelt runoff in late spring or early summer. High streamflow during spring runoff can reduce benthic algal abundance by physical disturbance through scouring or streambed reorganization (Power and Stewart, 1987). Streamflow also affects water-quality conditions such as water temperature, light availability, and nutrient loading and dilution (Rolls and others, 2012; Piggott and others, 2015; Schneider, 2015). Snowmelt-derived runoff is cold and has elevated turbidity, which limits light availability to benthic algae. Low streamflow during the summer can result in warmer temperatures favorable to algae. During years with earlier snowmelt runoff, warm water temperatures earlier in the summer can extend the growing season for algae, leading to higher accumulation of benthic algae (Schneider, 2015).

Using streamflow data from 1980 through 2020, significant decreases in August and September streamflow were identified for White River above Coal Creek (site 18; fig. 2). Decreases in May and June streamflows and corresponding increases in April may indicate a shift toward earlier snowmelt runoff (Day, 2023).

Field measurements were used to assess if the streambed moved during peak runoff in 2018, 2019, and 2020 using the modified critical shear method (U.S. Department of Agriculture Forest Service, 2008). Results indicated movement of the median particle size at zero sites in 2018, seven sites in 2019, and two sites in 2020. Results from the linear mixedeffects model show larger median particle sizes were associated with greater algal biomass. Large particles provide stable areas for algae to establish and can increase algal settlement rates by slowing the velocity of oncoming water (Burkholder, 1996).

Nutrients

Nutrients, including nitrogen and phosphorus, are essential substances promoting the growth and accumulation of algae. Nutrients can occur in the environment through weathering of rock or soil, breakdown of organic material, atmospheric deposition, and human activities (Hem, 1985; Murphy and Sprague, 2019; Stets and others, 2020).

Data from 2000 through 2020 for North Fork White River at Buford, South Fork White River at Buford, White River above Dry Creek, and White River above Coal Creek (sites 8, 12, 15, and 18; fig. 2) were used to assess seasonal patterns and changes in nutrient concentrations and loads. Phosphorus concentrations and loads increased through time, while nitrogen concentrations and loads decreased. A relatively uniform increase in total phosphorus between the North and South Forks of the White River indicates the factors contributing to the increases may be regional in scope. In fact, similar increases in phosphorus were observed nearby in the Yampa River (Day, 2021). Changes in the relative availability of nitrogen to phosphorus, represented by the molar ratio of nitrogen to phosphorus, show conditions in the North and South Forks of the White River changed from colimited to nitrogen limited starting around 2012. Results from the linear mixed-effects model show that more nitrogen-limiting conditions (greater phosphorus availability) corresponded to greater algal biomass.

A synoptic sampling was completed in October 2020 to identify the relative magnitude of nutrient inputs from potential source areas. The results represent nutrient loading conditions at the time of sampling and are not meant to explain the long-term changes in nutrients throughout the basin. This effort identified the lower North Fork subbasin as having a large phosphorus yield (load divided by subbasin area). Loads of total phosphorus and orthophosphate were elevated at North Fork White River below Marvine Creek (site 5; fig. 2). An elevated total nitrogen concentration and load occurred between the two most downstream sites on the White River (water-quality sites 19 and 20, fig. 2), likely driven by inputs of organic nitrogen.

Water Temperature

Variation in the magnitude and timing of snowmelt runoff led to interannual variation in water temperature. In general, Cladophora glomerata growth is promoted at water temperatures above a threshold of 13 degrees Celsius (Graham and others, 1982). The threshold was exceeded by early to mid-June in 2018, 2020, and 2021. The threshold was not reached until July in 2019, reflecting high streamflow and an extended period of snowmelt runoff. Results from the linear mixed-effects model indicate higher water temperatures were associated with greater benthic algal biomass; all water-quality sites with mean August water temperatures below the Cladophora glomerata threshold (dashed line, fig. 3A) had consistently low algal biomass (fig. 3D). Water temperatures were warmer at downstream water-quality sites, corresponding with higher algal biomass.

Riparian Canopy Cover

Riparian canopy cover affects the amount of sunlight reaching surface water. Low canopy cover, and therefore high light availability, promotes warmer temperatures and increased algal biomass (Warnaars and others, 2007). Canopy cover at water-quality sites in the upper White River Basin was low and ranged from 0 to 26 percent (fig. 3B).