Flaming Gorge Technical Working 

Group

April 25, 2018 Hydrology Summary

Jed Parker, P.E.

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 April

2018 mid-month forecast is 1,070,000 acre-feet (1,070 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 combined April Final Forecast of the Yampa River at Maybell and Little Snake at Lily 

is 755,000 acre-feet. The Colorado River Basin Forecast Center (CBRFC) Ensemble 

Streamflow Prediction (ESP) for the Yampa River (Maybell + Lily) is forecasting a runoff volume

of 819,000 acre-feet due to increased precipitation since the April Final Forecast was released. 

This ESP forecast would fall into the moderately dry hydrologic classification of the ROD. 

Snow water equivalent (SWE) as of April 24, 2018, for the Upper Green River and Yampa/White 

River Basins are 111 and 89 percent of median, respectively. Flaming Gorge SWE is similar to 

1987 and 1968, while Deerlodge SWE is similar to 2001 and 1955. The Tower snotel site, used 

as a reference point for Yampa snowpack and runoff, currently has 45 inches of SWE (91.3

percent of median). On April 24, 2001 the Tower SNOTEL measured a SWE of 39.6 inches 

during a runoff season where the Yampa yielded 775 kaf (64% of 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.
2

Basin Hydrology

Green River Basin Hydrology

The April 24, 2018, ESP forecast of April through July unregulated inflow (current forecast) for 

Flaming Gorge Reservoir is 1,060,000 acre-feet (AF) (108% of 30-year average). This forecast 

falls at 50 percent exceedance based on the historic unregulated inflow record (1963-2017). 

Figure 1 illustrates the Upper Green River SWE as of April 24, 2018 and compares it against the 

nearest April-July water years 2016 and 2005. 

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

Snow Water Equivalent (SWE), 2016, and 2005 percent of average SWE.

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. 
3

Figure 2 - Upper Green River and Yampa River Basins modeled SWE significant areas as of April 24, 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 April 24 ESP forecast of 1,060

kaf is 10,000 acre-feet lower and 60,000 acre-feet higher than the April mid-month and final 

forecasts, respectively. 
4

Figure 3 - Upper Green River Basin Water Supply Forecast as of April 24, 2018.

The analogous years unregulated inflow volumes based off of April 24, 2018 ESP are 2016 with 

total inflow into Flaming Gorge of 1,047 kaf (107 percent of average) and 2005 with total inflow 

into Flaming Gorge of 1,111 kaf (113 percent of 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 April mid-month April-July forecast for Flaming Gorge (1,060 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 Probable value of 1,060 kaf is taken from the mid-month April forecast while the 

Maximum and Minimum Probable values are taken from the April Final Forecast produced on 

April 1, 2018.
5

Figure 4 - Flaming Gorge Reservoir April mid-month forecast and ranked historic April-July unregulated inflow volume 

for years 1963-2017.

Figure 5 - Flaming Gorge Reservoir mid-month April forecast probability (percent exceedance) and historic April-July 

unregulated inflow volume for years 1963-2017.
6

Yampa River Basin Hydrology

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

the April 24, 2018 CBRFC ESP forecasts is 819 kaf (64.6% of 30-year average). This forecast 

falls at approximately 82% exceedance based on a ranking of the historic record (1922-2017). 

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of April 24, 2018 and compares 

it against the nearest April-July water year unregulated inflow volume in 2001. While the year 

1955 is also viewed as an analogous year to 2018 in terms of forecasted April-July runoff, 

snowpack data for 1955 is unavailable. The next highest year for which snowpack data is 

available is 1991, which is included in Figure 6.

Figure 6 - Yampa River above Deerlodge SNOTEL Group. 1981-2010 percent of average SWE compared 

against 2018 YTD, and analog years 2001 and 1991.

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 April 1 forecasting an April-July runoff of 750 kaf. The CBRFC April 

24, 2018 ESP forecast for the Yampa River at Deerlodge Park is 813 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 April 24, 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 Probable value of 819 is taken from the April 24, 2018 CBRFC ESP forecast 

while the Maximum and Minimum Probable values are taken from the April Final Forecast 

produced on April 1, 2018.
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Figure 8 - Yampa River Basin (Maybell plus Lily) April 24, 2018 ESP forecast and ranked April-July 

unregulated inflow volume for years 1922-2017.

Figure 9 - Yampa River – Maybell Plus Lily April 24, 2018 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 

wet 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 at 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 508

1966 679

1990 703

2007 736

1987 746

2001 790

MOST 819

1955 845

1959 852

1991 934

1953 938

1972 966

MAX 1,090
10

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. 

April 24, 2018 

ESP 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
12

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.

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. 
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Table 4

Larval Trigger Study Plan Table 2 Matrix

Record of Decision Spring Flow Objectives

This hydrologic update provides information regarding current conditions and how it relates to 

the Operating Plan detailed in the 2005 Flaming Gorge Final Environmental Impact Statement 

(FEIS) in Section 2.5.3.1. The operating plan 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. The operating plan is also to identify 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 be limited to those described for one classification lower or two classifications higher than 

the classification for the current year.” 

The potential classifications for 2017 are as follows:

Average (Above Median) Classification

The differences between the hydrology in the Upper Green and Yampa River Basins is extreme. 

Yampa River flows are not expected to provide the necessary days above 10,000 cfs to obtain 

the moderately wet classification targets. It is likely that the average (above median) 

classification will prevail. 
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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: 

Table 5 – 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

≥ 8,300 cfs in 50% of average 

years

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

average years

Flow Recommendations and FEIS

Average (Below Median) Classification

The differences between the hydrology in the Upper Green and Yampa River Basins is extreme. 

Yampa River flows are not expected to provide the necessary days above 10,000 cfs to obtain 

the moderately wet classification targets. It is likely that the average (above median) 

classification will prevail. 

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 6 – 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

≥ 8,300 cfs in 50% of average 

years

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

average years

Flow Recommendations and FEIS

Moderately Dry Classification

The differences between the hydrology in the Upper Green and Yampa River Basins is extreme. 

Yampa River flows are not expected to provide the necessary days above 10,000 cfs to obtain 

the moderately wet classification targets. It is likely that the average (above median) 

classification will prevail. 

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

≥ 8,300 cfs in 50% of average 

years

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

average years

Flow Recommendations and FEIS