Flaming Gorge Technical Working 

Group

April 24, 2017 Hydrology Summary

Heather Patno

For the purposes of discussions related to implementing the ROD in 2017, 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 2016. The April 1, 

2017 midmonth forecast was 2,260,000 acre-feet for Flaming Gorge. The hydrologic 

classification will be wet (<10% exceedance) for spring 2017. 

The combined April through July forecast of the Yampa River at Maybell and Little Snake at Lily 

is 910,000 acre-feet. This forecast would fall into the moderately dry hydrologic classification of 

the ROD. 

Snow water equivalent (SWE) as of April 6, 2017, for the Upper Green River and Yampa/White 

River Basins are 154 and 89 percent of median, respectively. Flaming Gorge SWE is similar to 

1986 and 1997, while Deerlodge SWE is similar to 1992 and 2001. The Tower snotel site, used 

as a reference point for Yampa snowpack and runoff, currently has 38.9 inches of SWE (85

percent of median). On April 6, Tower SNOTEL measured SWE inches of: 

• 2001 = 39.6 in; Yampa 790 kaf obs spring (62%)

• 2004 = 39.7 in; Yampa 678 kaf obs spring (53%)

Basin Hydrology

Green River Basin Hydrology

The April 1, 2017, midmonth forecast of April through July unregulated inflow (current forecast) 

for Flaming Gorge Reservoir is 2,260,000 acre-feet (AF) (231% of 30-year average). This 

forecast falls at < 5% exceedance based on the historic unregulated inflow record (1963-2016). 

Figure 1 illustrates the Upper Green River SWE as of April 24, 2017, and compares it against 

water years 1986 and 1997. Figure 2 indicates that the forecast for Flaming Gorge Reservoir 

has remained stable and the April 18, 2017, midmonth forecast remained unchanged at 2,260
2

kaf from two weeks ago. The ESP most probable value of 2.39 maf is considerably higher than 

the April midmonth forecast value of 2.26 maf. The CBRFC forecast includes uncertainty in 

modeled snow and soil moisture states that are reflected in the current forecast. 

Historic year unregulated inflow volumes that compare with current snowpack are 1986 with 

total inflow into Flaming Gorge of 2,222 kaf (maximum historic year) and 1997 with total inflow 

into Flaming Gorge of 1,668 kaf (170 percent of average).

Figure 3 illustrates the current forecast in relation to the historic unregulated inflow volumes. 

Figure 4 illustrates Flaming Gorge Reservoir April final forecast probability (percent 

exceedance).

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

2017 YTD Snow Water Equivalent (SWE) and 1986, 1997 and 2016 percent of average SWE
3

FIGURE 2. Upper Green River Basin Water Supply Forecast as of April 6, 2017.

FIGURE 3. Flaming Gorge Reservoir April 18, 2017 forecast and ranked historic April-July unregulated 

inflow volume for years 1963-2016
4

FIGURE 4. Flaming Gorge Reservoir April final forecast probability (percent exceedance) and historic 

April-July unregulated inflow volume for years 1963-2016

Yampa River Basin Hydrology

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

910,000 AF (70% of 30-year average). This forecast falls at approximately 76% exceedance 

based on a ranking of the historic record (1922-2016). 

Figure 5 illustrates the Yampa River at Deerlodge Park SWE as of April 24, 2017, and 

compares it against water years 2001 and 2004. Figure 6 indicates that the forecast for the 

Yampa River at Deerlodge is declining and the April 18, 2017, final forecast decreased from 

1,050 kaf to 910 kaf.1

 

Figure 7 below shows the current forecast in relation to historic flow volumes. Figure 8

illustrates the Yampa River at Maybell plus Lily April final forecast probability (percent 

exceedance). 

 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.
5

FIGURE 5. Yampa River above Deerlodge SNOTEL Group. 1981-2010 percent of average SWE 

compared against 2017 YTD, and analog years 2001, 2004 and 2016 percent of median SWE

FIGURE 6. Yampa - Deerlodge Park Water Supply Forecast as of April 24, 2017.
6

FIGURE 7. Yampa River Basin (Maybell plus Lily) April final forecast and ranked April-July unregulated 

inflow volume for years 1922-2016

FIGURE 8. Yampa River – Maybell Plus Lily April final forecast probability (percent exceedance) and 

historic April-July unregulated inflow volume for years 1963-2016
7

Probabilities of Flow Events for Spring 2017

The Flaming Gorge unregulated inflow and Yampa River forecasts are wet and average (below

median), respectively. The Green River Basin continues to trend wetter, while the Yampa River 

Basin is trending drier. An analysis was completed to assist in the determination of appropriate 

flow objectives for spring and summer 2017. The Flaming Gorge forecast is so high that ten 

similar between the minimum and maximum forecasts are unavailable. 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 

flow levels at Yampa River (Maybell plus Lily). The current analysis indicates that it is likely 

Yampa River flows above 10,000 cfs necessary to meet the wet or moderately wet duration 

targets in Reach 2 will not be achieved this year.

Table 1

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

Ten Similar Years to the April 18, 2017 Forecast

Thousand Acre-Feet (KAF)

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 765

2007 736

1987 746

2001 790

1955 845

1959 852

MOST 910

1991 934

1953 938

1972 966

1940 968

1946 976

MAX 1,370
8

Table 2

Spring 2017 – Days above Specific Flow Thresholds in the Yampa River 

(Maybell plus Lily)

Based on the April 1, 2017 Forecast

Percent Exceedance (%)

Colorado Basin River Forecast Center Yampa River Analysis

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

thirty years of historic temperature and precipitation data (1981-2010) using a modified Monte 

Carlo method to provide projections of flow. The RFC provides projections based upon (1) strict 

observance to the historic dataset and (2) incorporation of the five-day quantitative precipitation 

forecast (QPF). QPF is the expected amount of melted precipitation over the five-day climate 

forecast period. 

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 18. The first half of April 

has been dry with slightly above normal temperatures. There has been significant snowmelt in the 

basin and the current snow water equivalent is at 71% of median. Many of the SNOTEL locations 

below 9000ft have had significant melt or are already melted out. The remaining snow exists only 

at the highest elevations.

As a result of the snowmelt, streamflows have continued to increase across the basin. The 

current peak to date at the Yampa River- Deerlodge is 5,090 cfs on 4/17.

Current weather models are forecasting an unsettled weather pattern with chances for 

precipitation for rest of the week. A stronger and colder storm with a better chance for 

precipitation is forecast over the basin for Thursday and Friday. As a result of continued 

snowmelt and a chances for precipitation, the 10 day streamflow forecasts are indicating flows 

may reach ~7000-7500 cfs before decreasing back to current levels. 

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 

based on current information. Figures 9 and 10 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 18, 2017 

Forecast % Exceed

Days above 

6000 cfs

Days above 

8000 cfs

Days 

above 

10000 cfs

Days above 

11000 cfs

Days above 

12000 cfs

Days above 

13000 cfs

Days above 

16000 cfs

25% 33 21 5 1 0 0 0

50% 25 9 2 0 0 0 0

75% 19 4 0 0 0 0 0

90% 19 3 0 0 0 0 0

YAMPA
9

Table 3

Maximum Peak Daily Flow on the Yampa River at Deerlodge

Based on the April 5, 2017, RFC Analysis

Percent Exceedance (%)

FIGURE 9. RFC April 5, 2017 projection of Yampa River at Deerlodge flows exceeding thresholds for 

various percent exceedance for the 2017 April through July spring period.

CBRFC 

April 18, 2017 

Projections % Exceed

Maximum 

Daily 

Peak (cfs)

Number of 

Days to 

Peak from 

04/01/17

Number of 

Days 

Above 

10,000 cfs

Number of 

Days above 

14,000 cfs

Number of 

Days above 

16,000 cfs

10% 13,465 53 10 0 0 

25% 11,572 43 4 0 0

50% 9,179 35 0 0 0

75% 8,275 20 0 0 0

90% 7,211 4 0 0 0

YAMPA
10

FIGURE 10. RFC April 5, 2017 projection of Yampa River at Deerlodge flow exceeding thresholds for 

based on the historic observed, simulation of historic climate with current initial conditions and simulation 

of historic climate including the ten-day QPF with initial conditions.

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:

Wet

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

1,751,000 AF the hydrological classification would be wet. It is not recommended to operate 

Flaming Gorge within the wet hydrologic classification.

The ROD spring flow objectives for wet years are:
12

Table 5 – Wet Spring Flow Objectives

Reach Spring Peak Magnitude 

(cfs) Spring Peak Duration

Reach 1 ≥ 8,600 cfs That necessary to achieve duration target in 

Reach 2

Reach 2

≥ 26,400 cfs One day in wet years

≥ 22,700 cfs Two weeks (i.e., 14 days) in wet years

≥ 18,600 cfs Four weeks (i.e., 28 days) in wet years

Flow Recommendations and FEIS

Moderately Wet

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

1,751,000 AF the hydrological classification would be wet. However, the Yampa River flow 

volume forecast falls within the average (below median) classification. The Yampa River is 

necessary to meet wet and moderately wet targets. It is recommended to operate Flaming 

Gorge down one classification within the moderately wet hydrologic classification. 

It is unlikely that the moderately wet classification targets will be met with the current Yampa 

River forecast in the average (below median) classification. However, the Record of Decision 

flexibility limits the decrease in hydrologic classification to one below the unregulated inflow 

volume into Flaming Gorge Reservoir. 

The ROD spring flow objectives for moderately wet years are:

Table 6 -- Moderately Wet 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

≥ 20,300 cfs 1 day in moderately wet years

≥ 18,600 cfs 2 weeks (i.e. 14 days) in moderately wet years
13

Flow Recommendations and FEIS

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. 

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 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