Flaming Gorge Technical Working 

Group

May 8, 2018 Hydrology Summary

Jed Parker, P.E. and Heather E. Patno

This hydrologic update provides information regarding current conditions and fulfills the 

requirements detailed in the 2005 Flaming Gorge Final Environmental Impact Statement (FEIS) 

in Section 2.5.3 related to an operational plan for Flaming Gorge Dam (“Hydrology Summary”). 

The Hydrology Summary is to describe the current hydrologic classification of the Green River 

and Yampa River Basins, including the most probable runoff patterns for the two basins. 

For the purposes of discussions related to implementing the ROD in 2018, an evaluation has 

been made of the current hydrologic conditions in the Upper Green River (i.e. above Flaming 

Gorge Dam) and Yampa River Basins. The evaluation centered on the historical unregulated 

inflow statistics for Flaming Gorge Dam during the period from 1963 through 2017. The May 

2018 April-July forecast is 1,000,000 acre-feet (1,000 kaf) for Flaming Gorge, which falls in the 

average (below median) hydrologic classification. Information regarding Yampa River hydrology 

indicates that the hydrologic classification will likely be moderately dry (>70% and <90% 

exceedance) for spring 2018. 

The official Colorado River Basin Forecast Center (CBRFC) combined May Final Forecast of 

the Yampa River at Maybell and Little Snake at Lily is 841,000 acre-feet. This forecast would 

fall into the moderately dry hydrologic classification of the ROD. 

Snow water equivalent (SWE) as of May 7, 2018, for the Upper Green River and Yampa/White 

River Basins are 99 and 74 percent of median, respectively. Flaming Gorge SWE is similar to 

2003 and 2006, while Deerlodge SWE is similar to 2001 and 2017. The Tower snotel site, used 

as a reference point for Yampa snowpack and runoff, currently has 44.6 inches of SWE (87 

percent of median). On May 7, 2017 the Tower SNOTEL measured a SWE of 44.5 inches 

during a runoff season where the Yampa yielded 1,069 kaf (84% of 40-year average, Maybell + 

Lily).

The difference between the Tower SNOTEL figures and Yampa River observed spring volume 

differs significantly and needs to be considered when comparing any values this early in the 

runoff season.
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Basin Hydrology

Green River Basin Hydrology

The May final Forecast of the April through July unregulated inflow (current forecast) for Flaming 

Gorge Reservoir is 1,000,000 acre-feet (AF) (102% of 30-year average). This forecast falls at 

55 percent exceedance based on the historic unregulated inflow record (1963-2017). 

Figure 1 illustrates the Upper Green River SWE as of May 7, 2018 and compares it against the 

nearest SWE years 2006 and 2003. 

Figure 1 - Upper Green River Basin SNOTEL Tracking. 1981-2010 percent of median compared against 2018 YTD 

Snow Water Equivalent (SWE), and analog years 2006 and 2003.

Figure 2 shows the spatial extent of significant areas of modeled snow accumulation for the 

Upper Green River Basin and how this is incorporated into the water supply forecasts. 
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Figure 2 - Upper Green River and Yampa River Basins modeled SWE significant areas as of May 7, 2018.

The CBRFC Water Supply Forecast for the Upper Green River Basin is provided in Figure 3. 

This figure indicates that the forecast for Flaming Gorge Reservoir has remained fairly stable

across the snowpack season with a slight increase in April. The May forecast of 1,000 kaf (AprJul) is equal to what was forecasted in the April final Forecast. 
4

Figure 3 - Upper Green River Basin Water Supply Forecast as of May 7, 2018.

The analogous years unregulated inflow volumes (based off offrom the May final Forecast) are 

1987 with total inflow into Flaming Gorge of 982 kaf (100 percent of 30-year average) and 2015

with total inflow into Flaming Gorge of 1,035 kaf (106 percent of 30-year average). SWE analog 

years will differ from unregulated inflow volumes because of the inclusion of spring precipitation 

events separate from snowmelt in total runoff volumes. 

Figure 4 illustrates the May final Forecast (Apr-Jul) for Flaming Gorge (1,000 kaf) in relation to 

the historic unregulated inflow volumes (1963 through 2017). Figure 5 illustrates Flaming Gorge 

Reservoir April final forecast probability (percent exceedance). For both figures the most, 

minimum and maximum forecast values are taken from the May final Forecast.
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Figure 4 - Flaming Gorge Reservoir – May final Forecast and ranked historic April-July unregulated inflow volume for 

years 1963-2017.
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Figure 5 - Flaming Gorge Reservoir – May final Forecast probability (percent exceedance) and historic April-July 

unregulated inflow volume for years 1963-2017.

Yampa River Basin Hydrology

The combined current forecast for the Little Snake at Lily plus Yampa River at Maybell based on 

the May final Forecast is 841 kaf (66% of 30-year average). This forecast falls at approximately 

81% exceedance based on a ranking of the historic record (1922-2017). 

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of May 7, 2018 and compares it 

against the nearest SWE years 2001 and 2017.
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Figure 6 - Yampa River above Deerlodge SNOTEL Group. 1981-2010 percent of average SWE compared 

against 2018 YTD, and analog years 2001 and 2017.

Similar to what was presented in the Green River Basin Hydrology section, Figure 2 also 

demonstrates the extent of modeled SWE in the Yampa River Basin. Figure 7 indicates that the 

forecast for the Yampa River at Deerlodge Park has increased over the month of April with the 

final forecast, released on May 1, 2018 forecasting an April-July runoff of 815 kaf.

1

 

 1 The Yampa River at Deerlodge forecast volume differs from the Yampa River – Maybell Plus Lily 

volume. The historic gage record to calculate the Maybell plus Lily forecast volume is significantly longer 

than the Deerlodge dataset. The forecast volumes will be close, but the actual volume may differ due to 

routing in the CBRFC forecast model.
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Figure 7 - Yampa River Basin at Deerlodge Park Water Supply Forecast as of May 7, 2018.

Figure 8 shows the current forecasts in relation to historic flow volumes. Figure 9 illustrates the 

Yampa River at Maybell plus Lily April final forecast probability (percent exceedance). For both 

figures the most, minimum and maximum probable values are taken from the May final 

Forecast.
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Figure 8 - Yampa River Basin (Maybell plus Lily) May final Forecast and ranked April-July unregulated 

inflow volume for years 1922-2017.

Figure 9 - Yampa River – Maybell Plus Lily May final Forecast probability (percent exceedance) and 

historic April-July unregulated inflow volume for years 1963-2017.
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Probabilities of Flow Events for Spring 2018

The Flaming Gorge unregulated inflow and Yampa River forecasts can be classified as average 

dry and moderately dry, respectively. An analysis was completed to assist in the determination 

of appropriate flow objectives for spring and summer 2018. The ten most similar historic years 

for the Yampa River Basin (Maybell plus Lily) compared to the current forecast (Table 1) were 

analyzed assuming a normal distribution. 

Table 2 presents the percent exceedance of cumulative days greater than or equal to various 

unregulated inflow volumes for the Yampa River (Maybell plus Lily). The current analysis 

indicates that it is likely Yampa River flows above 10,000 cfs will not be achieved this year.

Table 1

Yampa River (Maybell plus Lily) – April through July Unregulated Volume

Ten Similar Years to the April 24, 2018 ESP Forecast

Thousand Acre-Feet (KAF)

Year

AprilJuly 

Unreg 

Inflow 

Volume 

(KAF)

MIN 651

1966 679

1990 703

2007 736

1987 746

2001 790

MOST 841

1955 845

1959 852

1991 934

1953 938

1972 966

MAX 1,055
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Table 2

Spring 2018 – Days above Specific Flow Thresholds in the Yampa River 

(Maybell plus Lily)

Based on the April 24, 2018 ESP Forecast

Percent Exceedance (%)

Colorado Basin River Forecast Center Yampa River Analysis

The Colorado Basin River Forecast Center (RFC) calculates exceedance probabilities based on 

thirty-five years of historic temperature and precipitation data (1981-2015) and current 

hydrologic conditions to provide projections of flow. The RFC provides projections based upon 

(1) strict observance to the historic dataset and (2) current hydrologic conditions including SWE, 

flow, and soil moisture and (3) incorporation of the five-day quantitative precipitation forecast 

(QPF). QPF is the expected amount of forecasted precipitation. 

The RFC provides a synopsis of the current seasonal outlook for the Yampa River at Deerlodge. 

This synopsis is provided below:

This outlook is based on flows from ESP with model states as of April 24. 

Month to date precipitation for April is above average in the Yampa River 

basin and below to near average in the Little Snake River basin. An active 

weather pattern has helped to improve snow and water supply conditions since 

the first of the month. There have been periods of warming and cooling which 

has resulted in periods of snowmelt at the low and mid elevations below 

10,000 ft. The most significant melt has occurred over the past two days.

Flows are expected to reach 4500-4700 cfs over the next two days from the 

most recent period of melt. 

The current weather forecasts are indicating a period of above average 

temperatures through Sunday before another storm system impacts the region. 

The details of the timing, strength, and forecasted precipitation of the 

storm are yet to be resolved. However, temperatures should return to near 

normal. Flows are expected to stay elevated between 3700-5000 cfs through the 

end of the month with additional snowmelt before gradually decreasing.

The RFC provides updated Yampa River April through July seasonal exceedance probabilities 

for both the river flows and daily maximum peak flow. Table 3 presents the RFC projections of 

maximum peak flow based on current information. Figure 10 and Figure 11 illustrate the 

probabilities of the Yampa River at Deerlodge river flows exceeding certain thresholds and 

exceedance probabilities over the April through July spring period. 

May final 

Forecast % Exceed

Days above 

8000 cfs

Days 

above 

9000 cfs

Days 

above 

10000 cfs

Days 

above 

11000 cfs

Days 

above 

12000 cfs

Days 

above 

13000 cfs

Days 

above 

14000 cfs

25% 14 8 2 0 0 0 0

50% 4 1 0 0 0 0 0

75% 2 0 0 0 0 0 0

90% 1 0 0 0 0 0 0

YAMPA
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Table 3

Maximum Peak Daily Flow on the Yampa River at Deerlodge

Based on the April 24, 2018, RFC Analysis

Percent Exceedance (%)

Figure 10 - RFC April 24, 2018 projection of Yampa River at Deerlodge flows exceeding thresholds for various 

percent exceedance at a weekly time step for the 2018 April through July period.

CBRFC 

April 24, 

2018 

Projections % Exceed

Maximu

m Daily 

Peak (cfs)

Number 

of Days to 

Peak 

from 

04/24/18

Number 

of Days 

Above 

4,000 cfs

Number 

of Days 

above 

5,000 cfs

Number 

of Days 

above 

6,000 cfs

Number 

of Days 

above 

8,000 cfs

Number 

of Days 

above 

10,000 cfs

Number 

of Days 

above 

12,000 cfs

10% 12,748 47 57 45 40 19 9 0 

25% 10,820 37 51 39 27 10 3 0

50% 9,198 30 43 30 18 3 0 0

75% 7,823 25 40 20 10 0 0 0

90% 7,184 7 30 15 8 0 0 0

YAMPA
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Figure 11 - CBRFC April 24, 2018 projection of Yampa River at Deerlodge maximum peak daily flow exceedance 

probabilities based on current hydrologic conditions and historical precipitation and temperature.

15-Day Yampa Streamflow Forecast

The CBRFC is indicating, based off of current modeling that the Yampa River will peak at some 

time between May 14 and May 17. The following, Figure 12 illustrates the forecasted flow 

values for the Yampa River, May 7 through May 17, 2018. The projected average flow values 

for each day in the Yampa at Deerlodge May 8 through May 22, 2018 are provided in .
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Figure 12 - Streamflow forecast for the Yampa River, May 7 - May 17, 2018.

Table 4

Deterministic forecast flow for Yampa-Derlodge, May 8 - May 22, 2018.

Based on May 7, 2018 CBRFC Modeling

Date

Average 

Daily Flow 

(cfs)

5/8/2018 5327

5/9/2018 6635

5/10/2018 7368

5/11/2018 7387

5/12/2018 7846

5/13/2018 7875

5/14/2018 8089

5/15/2018 6719

5/16/2018 5261

5/17/2018 4979

5/18/2018 5546

5/19/2018 5904

5/20/2018 5494

5/21/2018 5193

5/22/2018 5097
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Larval Trigger Study Plan Projected Operations

Previous to the implementation of the ROD, the U.S. Fish and Wildlife Service issued a 

Biological Opinion in 1992 that timed releases from Flaming Gorge to occur during the spring 

peak of the Yampa River. Under the ROD, Flaming Gorge releases are timed during the Yampa 

River spring peak and immediate post peak. 

The Larval Trigger Study Plan (LTSP) experimental protocol alters Flaming Gorge releases to 

occur after razorback sucker larvae have been observed in the Green River below Flaming 

Gorge Dam. This modification in timing of spring peak releases alters projected operations from 

Flaming Gorge as compared against historic releases. 

Table 4 outlines the LTSP matrix used to determine the peak flow measured at Jensen, targeted 

study wetlands, number of days exceeded for each hydrologic classification. 

Table 5

Larval Trigger Study Plan Table 2 Matrix

Record of Decision Spring Flow Objectives

The Hydrology Summary identifies the most likely Reach 2 flow magnitudes and durations that 

are to be targeted for the upcoming spring release. It further specifies that “[b]ecause 

hydrologic conditions often change during the April through July runoff period; the operations 

plan would contain a range of operating strategies that could be implemented under varying 

hydrologic conditions. Flow and duration targets for these alternate operating strategies would 
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be limited to those described for one classification lower or two classifications higher than the 

classification for the current year.” 

The potential classifications for 2018 are as follows:

Average (Below Median) Classification

If the April through July unregulated inflow into Flaming Gorge Reservoir remains in the range 

from 789,000 AF to 1,060,000 AF the hydrological classification would be average (below 

median). The Yampa River hydrology falls within the moderately dry hydrologic classification 

and current analysis indicates it is unlikely the Yampa River will achieve flow durations sufficient 

to meet average (below median) target thresholds at Jensen. It is therefore recommended to 

operate Flaming Gorge within the moderately dry hydrologic classification as described further 

below.

The peak flow as measured at Jensen, Utah, would correspond with the average (below

median) hydrologic condition with targeted flows between 14,000 and 18,600 cfs for a period 

between 1 to 14 days in Reach 2. These flows provide connection at the Stewart Lake, Above 

Brennan, Old Charley Wash, Thunder Ranch, Bonanza Bridge, Johnson Bottom, Stirrup and 

Leota 7 floodplains.

ROD spring flow objectives for average (below median) years are:

Table 6 – Average (Below Median) Spring Flow Objectives

Reach

Spring Peak 

Magnitude

(cfs)

Spring Peak Duration

Reach 1 ≥ 4,600 cfs That necessary to achieve duration target in 

Reach 2

Reach 2 ≥ 8,300 cfs in 50% of 

average years

One week (i.e. 7 days) in 50% of average 

years

Flow Recommendations and FEIS

Average (Above Median) Classification

If conditions become wetter than the current forecast at Flaming Gorge Reservoir and the April 

through July forecast increases between 1,060,000 AF and 1,351,000 AF, the hydrological 

classification would be average (above median). 

The peak flow as measured at Jensen, Utah, would correspond with the average (above 

median) hydrologic condition with targeted flows between 18,600 and 20,300 cfs for a period 

between 1 to ≥14 days in Reach 2. These flows provide connection at the Stewart Lake, Above 

Brennan, Old Charley Wash, Thunder Ranch, Bonanza Bridge, Johnson Bottom, Stirrup and 

Leota 7 floodplains.

ROD spring flow objectives for average (above median) wet years are: 
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Table 7 – Average (Above Median) Spring Flow Objectives

Reach

Spring Peak 

Magnitude

(cfs)

Spring Peak Duration

Reach 1 ≥ 4,600 cfs That necessary to achieve duration target in 

Reach 2

Reach 2 ≥ 18,600 cfs in 50% 

of average years

Two weeks (i.e. 14 days) in 25% of all 

average years

Moderately Dry Classification

If the April through July Yampa River flow remains in the range from 677,000 AF to 989,000 AF 

the hydrological classification would be moderately dry. It is recommended to operate Flaming 

Gorge within the moderately dry hydrologic classification.

The peak flow as measured at Jensen, Utah this year would correspond with the moderately dry 

hydrologic condition. The LTSP outlines moderately dry flows between 8,300 cfs and 14,000 

cfs at Jensen for a period between 7 to 14 days, and minimum seven-day duration above 8,300 

cfs during larval drift. These flows provide connection at Stewart Lake, Above Brennan and Old 

Charley Wash. 

The ROD spring flow objectives for moderately dry years are:

Table 8 – Moderately Dry Spring Flow Objectives

Reach Spring Peak Magnitude

(cfs) Spring Peak Duration

Reach 1 ≥ 4,600 cfs That necessary to achieve duration 

target in Reach 2

Reach 2 ≥ 8,300 cfs 1 week (i.e. 7 days)

Flow Recommendations and FEIS