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Water Resources Monitoring in the Snake River Above and Below Jackson Lake, 2023

Grand Teton National Park

The Greater Yellowstone Inventory and Monitoring Network monitors water quality and analyzes river discharge (flow) in the Snake River from spring through fall each year above and below Jackson Lake. The Snake River has high water quality at these sites and is valued for its natural, cultural, and recreational resources. In this article, we report our 2023 water resources monitoring results.

A slightly rippling blue river reflecting the blue sky, fluffy clouds, and conifers scattered on both sides of the river.
Snake River at Flagg Ranch, Wyoming

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The Snake River Monitoring Sites

The Snake River is the largest tributary of the Columbia River. It originates in western Wyoming and flows through Idaho and Oregon before converging with the Columbia River in Washington State. Its drainage basin also includes parts of Utah and Nevada.and below Jackson Lake, a dam-regulated lake. The Greater Yellowstone Network samples the Snake River at two monitoring sites: above Jackson Lake at Flagg Ranch, Wyoming, and below Jackson lake at Moose, Wyoming (Figure 1).

A map of the Snake River flowing through Grand Teton and Yellowstone National park with Moose, WY, marked south of Jackson lake and Flagg Ranch, WY, north of Jackson Lake.
Figure 1. Map of the watersheds, major tributaries, and National Park Service lands associated with the Snake River monitoring sites at Flagg Ranch and Moose, Wyoming.

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By the time the Snake River reaches Flagg Ranch (Figure 2), it has a drainage area of 486 mi2 (1259 km2) and runs west and then south for 47 miles (76 km) from its source on the Two Ocean Plateau to Jackson Lake. The headwaters flow through Yellowstone National Park, Bridger-Teton National Forest, and then into the John D. Rockefeller, Jr. Memorial Parkway, which is co-managed with Grand Teton National Park. Major tributaries include Heart River and Lewis River, both of which originate in Yellowstone National Park from lakes bearing the same names. The entirety of the watershed for this river section is located within National Park Service and US Forest Service lands.

The Snake River continues west and south from Flagg Ranch for about 7 miles before flowing into Jackson Lake. The Jackson Lake Dam exerts considerable influence on the Snake River for about 4.5 miles until Pacific Creek flows into the Snake River. Farther down, Buffalo Fork and Spread Creek tributaries also provide significant contributions to the Snake River. Immediately below the dam before these tributaries, dam operations can reduce peak flow by half the estimated unregulated flow, and median date of maximum flow can be delayed by over three weeks, which can have a significant impact on the hydrologic system. The entire Snake River watershed upstream of Moose (Figure 3) occurs on lands administered by the National Park Service and US Forest Service. All river segments are managed under the Snake River Headwaters Comprehensive River Management Plan, as directed by the Craig Thomas Snake Headwaters Legacy Act of 2008 (PL111-11), a 2009 amendment to the 1968 Wild and Scenic Rivers Act.

A calm, blue river with mostly flat, snowy banks in a snowy landscape of scattered conifers under a deep blue sky.
Figure 2. Looking downstream from the Flagg Ranch, Wyoming, monitoring location on the Snake River in April 2023.

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A rippling dark blue river flowing through conifer lined banks and around a small, raised island in the distance.
Figure 3. Looking downstream from the bridge over the Snake River at the Moose, Wyoming, monitoring location in September 2023.

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Water Flow in the Snake River

The Flagg Ranch and Moose, WY, locations were chosen as long-term water monitoring sites because they are co-located with US Geological Survey (USGS) gaging stations. The stream gage on the Snake River at Flagg Ranch (USGS gage 13010065) has daily flow data dating back to October 1983, while the stream gage at Moose (USGS gage 13013650) has daily flow data starting in 1995.

The 2023 hydrograph for the Snake River at Flagg Ranch (Figure 4 left) exhibits a general pattern of high early summer flows and lower baseflows in late summer and fall that are common in snowpack driven systems of the Northern Rockies. The average annual peak flow between 1984 and 2022 was 6,570 cubic feet per second (cfs) and occurred on 26 May (day 146 of the year). Minimum annual daily flows over this same period of record averaged 232 cfs. In 2023, a peak discharge of 6,260 cfs occurred on 24 May, which matches the average historical hydrographs as well. Base flows during the fall and winter tracked with the historical averages. Minimum flows in 2023 were 231 cfs, and the lowest flows occurred in January and February.

In 2023, the Snake River at Moose, WY, had summer discharge values well below the 1995–2022 averages and only began to follow historical trends in early August (Figure 4 right). The average annual peak flow at the Moose location from 1995–2022 was 12,165 cfs and the average date of peak flow was 6 June (day 157 of the year). In 2023, the daily maximum discharge was 6,980 cfs, just over half of the historical average daily flow. Peak daily flows in 2023 occurred on 25 May, 11 days before historical peak flows. Minimum flows on the Snake River at Moose, WY, in 2023 were 662 cfs, and the lowest flows predominantly occurred at the end of March and early April.

Two line graphs of water flow showing high early summer flows and lower baseflows in late summer and fall that were similar to historical averages at Flagg Ranch and below average discharge values through July at Moose.
Figure 4. Snake River flow hydrographs at Flagg Ranch (left) and Moose (right), Wyoming. The blue, solid line represents the 2023 discharge data, while the black, dashed line is the period of record averages. The shaded area represents the 25th to 75th percentile of discharge based on historical records, which would be considered a “normal flowˮ record. Note the y-axis scale and period of record are different for these two locations.

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Water Quality in the Snake River

Water quality in the Snake River headwaters is considered very high quality and the river is valued for its natural, cultural, and recreational contributions to the region. The Greater Yellowstone Network sampled both Snake River sites during eight months of the year (February–November, except March and July). Each event includes collecting core water quality parameters—including temperature, specific conductance, dissolved oxygen, pH, and turbidity— and water samples for water chemistry.

One important metric for both recreation and wildlife is stream temperature. The maximum daily water temperatures in the Snake River exceeded the Wyoming Department of Environmental Quality surface water quality standards (WY DEQ 2018) guidance for cold water fish temperature (20°C or 68°F) at Flagg Ranch, WY, on 40 days throughout July and August (Figure 5 left). At Moose, WY, maximum daily water temperatures occurred on 10 days at the end of July and mid-August (Figure 5 right).

Two line graphs of discharge and temperature showing maximum daily water temperatures in the Snake River exceeded standards for cold water fish for 40 days in July and August at Flagg Ranch and on 10 days at Moose.
Figure 5. Daily discharge (in dfs, solid blue line) and maximum daily water temperatures (°C; dotted red line) from Flagg Ranch (left) and Moose (right), Wyoming. The Wyoming DEQ cold water fish criterion is represented by the horizontal gray dashed line at 20°C.

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Water quality results in 2023 were comparable to previous years and indicative of high-quality waters; samples contained low levels of dissolved and total nutrients. Trace metals such as arsenic, copper, and iron have been detected in the watershed and are naturally present in measurable concentrations. Fluctuations in the trace metals are driven by discharge during run-off. Iron and Phosphorus concentrations increase with more run-off and were detected at the highest levels during peak flows. Total iron levels exceeded the Wyoming Chronic Aquatic Life Criterion for total recoverable iron (1 mg/L) during our May sampling trip at both Flagg Ranch and Moose (Figure 6 shows Flagg Ranch).

Line graph showing that total iron levels exceeded the standard in May at Flagg Ranch.
Figure 6. Daily discharge (in cfs, solid blue line) shown with concentrations(mg/L) of total iron (Fe; red circles), collected during the 2023 monthly sampling at the Flagg Ranch monitoring location. The Wyoming Chronic Aquatic Life Criterion is represented by the horizontal gray dashed line at 1.0 mg/L total extractable iron.

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2023 Snake River Water Quality Data Tables

The following tables show core water quality field results for the Snake River at Flagg Ranch, WY (Table 1a), and the Snake River at Moose, WY (Table 1b), from sampling events conducted by the Greater Yellowstone Network during calendar year 2023. Table 2a and Table 2b show the corresponding water chemistry lab data for Flagg Ranch and Moose, respectively. All Data for current and previous years can be accessed in the NPS Data Store. Data can also be downloaded from the Water Quality Portal using “11NPSWRD_WQX-GRTE_SNR01ˮ for Flagg Ranch and “11NPSWRD_WQX-GRTE_SNR02ˮ for Moose, WY as the SiteID in the “Advancedˮ menu. Of note, some 2023 sampling events did detect trace amounts of total phosphorous in some of the blank quality control samples (see asterisks [*] next to total phosphorus values in Table 2a and Table 2b for dates when this occurred). While always less than the actual amount detected and very low overall, please use caution when considering total phosphorous values and contact the Greater Yellowstone Network for more details if needed.
Table 1a. Water quality field data for the Snake River at Flagg Ranch, WY. These samples were collected onsite using a YSI Exo1 sonde, and each value represents an average of four measurements taken across the stream sampling width. 

Table 1b. Water quality field data for the Snake River at Moose, WY. These samples were collected onsite using a YSI Exo1 sonde, and each value represents an average of four measurements taken across the stream sampling width. 
Table 2a. Water chemistry lab results (in mg/L) for the Snake River at Flagg Ranch, WY. All samples were processed at Energy Laboratories in Billings Montana. For results with a less than symbol (e.g., < 0.05), the number represents the reporting limit (in mg/L), which is the threshold value that many analytical labs consider to be the lowest reportable value for an individual analyte. The reporting limit may be higher than the detection limit, and the analyte may be present in the sample but at concentrations less than the lab reports on. Sampling events include regular samples (Reg) and replicate samples (Rep) for comparison. Field blanks were also performed and were processed using certified inorganic free deionized water. All field blank samples were below the detection limit and are not reported here unless noted. Asterisks (*) are added to total phosphorus values when there were trace amounts of total phosphorus in some of the blank quality control samples collected on that date. 
Table 2b. Water chemistry lab results (in mg/L) for the Snake River at Moose, WY. All samples were processed at Energy Laboratories in Billings Montana. For results with a less than symbol (e.g., < 0.05), the number represents the reporting limit (in mg/L), which is the threshold value that many analytical labs consider to be the lowest reportable value for an individual analyte. The reporting limit may be higher than the detection limit, and the analyte may be present in the sample but at concentrations less than the lab reports on. Sampling events include regular samples (Reg) and replicate samples (Rep) for comparison. Field blanks were also performed and were processed using certified inorganic free deionized water. All field blank samples were below the detection limit and are not reported here unless noted. Asterisks (*) are added to total phosphorus values when there were trace amounts of total phosphorus in some of the blank quality control samples collected on that date. 

Monitoring Methods

For in-depth methods, please see the protocol links provided at the bottom of this section. In brief, the Greater Yellowstone Inventory and Monitoring Network collects water samples monthly durring ice-free periods generally following depth- and width-integrated protocols outlined in the US Geological Survey (USGS) National Field Manual for the Collection of Water-Quality Data.

Samples are usually collected from a bridge-board or crane, reel, and DS-95 suspension sampler (Figure 7). In wadeable depths and low flow, we use a 1 L, hand-held DH-81 sampler affixed to a 1 m wading rod. At multiple locations along a cross section of the river we collect water using vertically integrated sampling techniques. Samples from the 1 L bottle are mixed into an 8 L churn splitter; we use the churn splitter to homogenize and dispense a representative subsample into laboratory-provided bottles. These bottles are then shipped overnight to an EPA-certified commercial lab for processing. Data undergo rigorous review and are then published using NPStoret and are publicly available online at the Water Quality Portal using the siteID, “11NPSWRD_WQX-GRTE_SNR01ˮ for Flagg Ranch, WY, and “11NPSWRD_WQX-GRTE_SNR02ˮ for Moose, WY.

Discharge (river flow estimates) and water temperature data for the Snake River are available online from the USGS National Water Information System. For the Snake River above Jackson Lake at Flagg Ranch, WY, data are listed under USGS station 13010065, and for the Snake River at Moose, WY, data are listed under USGS station 13013650.

Read the full protocols and standard operating procedures for water quality and discharge on the NPS Datastore.

Two scientists standing on a bridge over a river turning a crank on a crane to lower a reel with equipment for collecting water samples into the river.
Figure 7. A Scientists in Parks intern working with the Greater Yellowstone Network operates a reel and crane to lower the DS-95 sampler (white “whale” suspended at bottom right) down to the water to collect depth- and width-integrated samples from the bridge at Moose, WY, during a June 2023 sampling event. Meanwhile, a Montana State University Intern looks upstream to check for recreational traffic and debris and helps collect the samples.

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Water Quality Criteria

The upper Snake River, located wholly within National Park Service and US Forest Service boundaries, is designated as Outstanding or Class 1 waters by the State of Wyoming. In Wyoming, this designation indicates that high quality waters are known to support fish or supply drinking water and no further degradation by point source discharges other than from dams is allowed. The Snake River is also protected by the Craig Thomas Snake Headwaters Legacy Act of 2008 (PL 111-11), a 2009 amendment to the Wild and Scenic Rivers Act of 1968.

Water quality monitoring results from the Snake River at Flagg Ranch and Moose, WY, are compared to the following federal and state water quality standards:

Authors: Jana Cram, Abigal Volk, Ben LaFrance, Tani Hubbard

Please cite this report as
Cram, J., A. Volk, B. LaFrance, and T. Hubbard. 2024. Water Resources Monitoring in the Snake River Above and Below Jackson Lake, 2023. Greater Yellowstone Network, National Park Service, Bozeman, Montana.

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Last updated: November 15, 2024