Yellowstone River water quality at Corwin Springs in 2021: low nutrients, natural arsenic

The Yellowstone River has relatively high water quality, and much of the watershed above the monitoring site is managed as federal land. The National Park Service's Greater Yellowstone Inventory and Monitoring Network sampled it at Corwin Springs, Montana, eight times from April to November 2021 for water chemistry and core parameters: temperature, specific conductance, dissolved oxygen, pH and turbidity.

Nutrients and sediment

Dissolved nutrients were low. Ammonia as nitrogen was below detection on every date except 8 April and 18 November 2021, when it measured 0.07 and 0.06 mg/L against a reporting limit of 0.05 mg/L. Nitrate plus nitrite as nitrogen ran between 0.03 and 0.22 mg/L, averaging 0.13 mg/L, with a reporting limit of 0.01 mg/L.

Total suspended solids and total phosphorus tended to rise with flow. Apart from a spike in suspended solids on 18 November, both peaked on the 14 June sampling date.

Daily discharge of the Yellowstone River at Corwin Springs in 2021 (solid line) with total suspended solids from the monthly samples (circles). Figure from the National Park Service.

Daily discharge in 2021 (solid line) with total phosphorus from the monthly samples (circles). Figure from the National Park Service.

Arsenic

Trace metals such as arsenic, zinc, mercury and lead are detected in Yellowstone National Park waters and are often naturally present. Total arsenic tended to be lower at higher flows. In 2021 it stayed below Montana's chronic aquatic life criterion (0.15 mg/L) but above the human health surface water standard (0.010 mg/L). Because the arsenic originates naturally in the park, Montana's environmental quality department set a separate annual median standard for nonanthropogenic arsenic for this river, 0.028 mg/L on the segment from the Montana-Wyoming border to Mill Creek, where Corwin Springs lies.

Daily discharge in 2021 with total arsenic concentrations. The dotted line is the nonanthropogenic annual mean flow criterion (0.028 mg/L), the solid line the chronic aquatic life criterion (0.15 mg/L) and the dashed line the human health criterion (0.010 mg/L). Figure from the National Park Service.

Temperature

The USGS records water temperature continuously at the same station. Daily average temperatures in 2021 were higher, and warmed earlier, than the average of the previous five years (2016-2020), and daily maximums were above those of 2016-2020. The 2021 peak of 22.3°C, on 1 August (day 213), exceeded the 2016-2020 average daily maximum of 20.6°C.

That matters for anglers: Montana Fish, Wildlife and Parks closes cold-water fishing when the temperature exceeds 73.0°F (22.8°C) at any time of day for three consecutive days. The 2021 peak stayed below that line.

Average daily water temperature in 2021 and for 2016-2020. Figure from the National Park Service.

Daily maximum water temperature in 2021 and 2016-2020. The dashed line marks the 22.8°C (73°F) level at which Montana's drought policy may close angling for three consecutive days. Figure from the National Park Service.

More data

Monthly lab results and field parameters for 2021 are in tables on the NPS page and in the Park Service's data store; earlier years' data are in the Water Quality Portal. The lab work was done by a commercial laboratory in Billings, Montana.

Sources

• National Park Service, "Water Quality in the Yellowstone River at Corwin Springs, Montana" (nps.gov article by the NPS Greater Yellowstone Inventory and Monitoring Network, part of a 2021 water monitoring series); rewritten in hubnx's own words.