The Yellowstone River at Corwin Springs in 2022: A Record Flood and Its Water Quality

Looking upstream from the monitoring site on the Yellowstone River at Corwin Springs, Montana, October 2022. Credit: National Park Service.

Each year the National Park Service's Greater Yellowstone Inventory and Monitoring Network monitors water quality and flow in the Yellowstone River at Corwin Springs, Montana. The river's water quality is relatively high, and it is ecologically important and a popular cold-water fishery.

The river and the site

At 1,080 km (671 miles), the Yellowstone — a tributary of the Missouri — is the longest undammed river in the lower 48 states. It rises in the Absaroka Range, its North Fork on Younts Peak, Wyoming, and flows northwest through Yellowstone National Park, feeding and draining Yellowstone Lake and passing over Yellowstone Falls and through the Grand Canyon of the Yellowstone, before leaving the park near Gardiner, Montana. Downstream it supplies irrigation water to farms and drinking water to towns including Laurel and Billings, Montana, and others east into North Dakota.

The monitoring station is 12 km (7.5 miles) downstream of Gardiner and the park boundary. Upstream of it, two segments were listed under Section 303(d) in Montana's 2018 Water Quality Integrated Report:

• a 14 km (8.7-mile) segment inside the park, from the Wyoming border to the park boundary — listed for ammonia, copper, nitrate plus nitrite, sediment, and arsenic above state drinking water standards;
• a 7.7 km (4.8-mile) segment from the park boundary to Reese Creek.

Figure 1. The watershed above the Corwin Springs monitoring site. Credit: National Park Service.

Figure 2. The monitoring site (yellow star) relative to the park boundary and Gardiner. Credit: National Park Service.

Flow: the 2022 flood

The U.S. Geological Survey's gage at Corwin Springs (06191500) has daily flow records from 1890, with a gap from 1894 to 1910. The river's flow is driven by snowmelt, peaking in spring.

In June 2022 the Yellowstone and its tributaries, including those across the park's Northern Range, had a 500-year flood. Flows tracked the long-term mean until mid-June, when heavy rain and warmer nights sped snowmelt on already saturated soil. The flood peaked 17,000 cfs above the previous record since 1890.

In the ten weeks before the flood, from April to June, the area had at least double its 1991–2020 average precipitation; July had 50% less. Flows returned to the historical average by July and fell below it by August.

Figure 3. Average daily discharge at Corwin Springs in 2022 and for 1890–2021, with the 25th and 75th percentiles. Credit: National Park Service.

Figure 4. Monthly air temperature and precipitation departures from 1991–2020 averages at Mammoth, Montana, in 2022, made with ClimateAnalyzer. Credit: National Park Service.

Looking downstream at the monitoring site in June 2022. Credit: National Park Service.

Water quality

Much of the watershed above the site is federal land. The network sampled the river eight times between March and November 2022 for chemistry and for temperature, specific conductance, dissolved oxygen, pH and turbidity.

• Nutrients were low. Ammonia (as nitrogen) was below detection except on March 23 (0.11 mg/L) and November 21 (0.18 mg/L), against a reporting limit of 0.05 mg/L. Nitrate plus nitrite (as nitrogen) ranged from under 0.01 to 0.28 mg/L and averaged 0.11 mg/L.
• Sediment and phosphorus rise with flow. Total suspended solids and total phosphorus peaked in the June 16 sample, three days after the flood.
• After the flood, extra tests on June 28 found total petroleum hydrocarbons — compounds from crude oil, including those in gasoline — below the 1 mg/L reporting limit. E. coli was present at 10 colony-forming units per 100 mL, a level the Montana Department of Environmental Quality regards as unsafe; E. coli can indicate contact with human or animal sewage.

Figure 5. Daily discharge in 2022 with total suspended solids from monthly samples. Credit: National Park Service.

Figure 6. Daily discharge in 2022 with total phosphorus from monthly samples. Credit: National Park Service.

Arsenic

Trace metals such as arsenic, zinc, mercury and lead occur naturally at measurable levels in the park's waters, and total arsenic tends to fall as flow rises. In 2022 arsenic at Corwin Springs stayed below Montana's chronic aquatic-life criterion (0.15 mg/L) but exceeded the human-health surface water standard (0.010 mg/L). Because the arsenic comes naturally from Yellowstone, Montana has set a separate annual median standard for natural arsenic in the river: 0.028 mg/L for the segment from the Montana–Wyoming border to Mill Creek, which includes the monitoring site.

Figure 7. Daily discharge in 2022 and total arsenic, with Montana's natural-arsenic criterion (0.028 mg/L), aquatic-life chronic criterion (0.15 mg/L) and human-health criterion (0.010 mg/L). Credit: National Park Service.

Temperature

The USGS records water temperature continuously at the station. Daily averages in 2022 followed the previous five years' until late July, then rose above them. The year's highest temperature, 21.9 °C on August 11, was 1 °C above the 2017–2021 average daily maximum of 20.9 °C. Montana Fish, Wildlife and Parks may close cold-water fishing when water reaches 73.0 °F (22.8 °C) at any time of day for three days in a row, or when flows fall below minimum levels.

Figure 8. Average daily water temperature in 2022 compared with previous years. Credit: National Park Service.

Figure 9. Daily maximum water temperature in 2022 compared with previous years; the dashed line marks the 22.8 °C threshold for angling closures. Credit: National Park Service.

How the samples are taken

The network samples monthly while the river is free of ice, following the USGS National Field Manual for the Collection of Water-Quality Data. In deep water, samples are taken from a bridge with a suspended sampler on a crane and reel; at wadeable depths and low flow, with a 1-litre hand-held sampler on a 1-metre wading rod. Water collected across the river is mixed in an 8-litre churn splitter, divided into laboratory bottles and shipped overnight to an EPA-certified lab (Energy Laboratories, Billings). Field readings average four measurements across the stream. The 2022 lab and field results are in the NPS Data Store and the Water Quality Portal (site 11NPSWRD_WQX-YELL_YS549.7M); flow and temperature are on the USGS National Water Information System.

A crane, reel and suspended sampler are used when the river cannot be waded. Credit: National Park Service.

A handheld multi-parameter instrument measures temperature, specific conductance, dissolved oxygen, pH and turbidity; photographed on the Lamar River. Credit: National Park Service.

Standards

Under the Clean Water Act, Yellowstone National Park's surface waters are Outstanding National Resource Waters, and Montana classifies them A-1, Outstanding Resource Waters. Leaving the park at Gardiner, the river is classified B-1: suitable, after conventional treatment, for drinking; for full-contact recreation; for salmonid fish and associated aquatic life, waterfowl and furbearers; and for farm and industrial water. Results are compared with EPA national criteria and Montana's standards.

Sources

• National Park Service, Greater Yellowstone Inventory and Monitoring Network, 2024, "Water Resources Monitoring in the Yellowstone River at Corwin Springs, Montana, 2022." https://www.nps.gov/articles/000/yellowstone-river-monitoring-2022.htm
• The article cites Montana Department of Environmental Quality reports on the river's natural arsenic (2019, 2020), Elliott and Hektner (2000), Planer-Friedrich and others (2007), and "When the River Breaks" in Park Science (2022).
• The source names the crane sampler "DS-95" in its text and "DH-95" in a caption, so no model is given here; and it says the river "exists" the park (exits).
• Rewritten in hubnx's own words.