Flaming Gorge Technical Working 

Group

April 11, 2016 Hydrology Summary

Heather Patno

For the purposes of discussions related to implementing the ROD in 2016, 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 2015. Based on 

these statistics and the April 1, 2016 final forecast of 740,000 acre-feet for Flaming Gorge, the 

hydrologic classification will be moderately dry (70% to 90% exceedance) for spring 2016. 

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

is 1,130,000 acre-feet. This forecast would fall into the average (below median) hydrologic 

classification of the ROD. 

Snow water equivalent (SWE) as of April 11, 2016, for the Upper Green River and Yampa/White 

River Basins are 93 and 97 percent of median, respectively. Flaming Gorge SWE is similar to 

1995 and 2005, while Deerlodge SWE is similar to 2000 and 2010. 

Snowstorms over the last few weeks assisted in accumulation in the Upper Green and Yampa 

River Basins to a greater extent than other areas in the Colorado River Basin system. The 

Upper Green snowpack, however, accumulated in noncontributing or less contributing runoff 

areas and the forecast is still below average at 76 percent of average. The Yampa River Basin 

did see significant snow accumulation that will assist in runoff. Weather patterns between now 

and May will determine the peak flows seen on the Green River at Jensen, Utah.

Basin Hydrology

Green River Basin Hydrology

The April 1, 2016, final forecast of April through July unregulated inflow (current forecast) for 

Flaming Gorge Reservoir is 740,000 acre-feet (AF) (76% of 30-year average). This forecast 

falls at approximately 72% exceedance (an increase from 77% exceedance) based on the 

historic unregulated inflow record (1963-2015). 
2

Figure 1 illustrates the Upper Green River SWE as of April 7, 2016 and compares it against 

water years 1995 and 2007. Figures 2 and 3 show 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. Figure 3 indicates that the forecast for Flaming Gorge Reservoir 

has increased and the April 11, 2016, estimated forecast has increased from 740 kaf to 874 kaf 

from one week ago. 

Figure 4 illustrates the current forecast in relation to the historic unregulated inflow volumes. 

Figure 5 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 

2016 YTD Snow Water Equivalent (SWE) and 1995, 2005 and 2015 percent of average SWE
3

FIGURE 2. Upper Green River Basin modeled SWE significant areas as of April 8, 2016.

FIGURE 3. Upper Green River Basin Water Supply Forecast as of April 8, 2016.
4

FIGURE 4. Flaming Gorge Reservoir April final forecast and ranked historic April-July unregulated inflow 

volume for years 1963-2015

FIGURE 5. Flaming Gorge Reservoir April final forecast probability (percent exceedance) and historic 

April-July unregulated inflow volume for years 1963-2015
5

Yampa River Basin Hydrology

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

1,130,000 AF (91% of 30-year average). This forecast falls at approximately 58% exceedance 

(up from 66% exceedance) based on a ranking of the historic record (1922-2015). 

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of April 8, 2016 and compares it 

against water years 2000 and 2010. 

Figures 7 and 8 show the spatial extent of significant areas of modeled snow accumulation for 

the Yampa River Basin and how this is incorporated into the water supply forecasts. Figure 8 

indicates that the forecast for the Yampa River at Deerlodge has continued to decrease and the 

April 11, 2016, estimated forecast has decreased from 1,130 kaf to 1,059 kaf from one week 

ago.1

 

Figure 9 below shows the current forecast in relation to historic flow volumes. Figure 10 

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

exceedance). 

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

compared against 2016 YTD, and analog years 2000, 2010 and 2015 percent of median SWE

 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.
6

FIGURE 7. Yampa River Basin modeled SWE significant areas as of April 8, 2016.

FIGURE 8. Yampa – Deerlodge Park Water Supply Forecast as of April 8, 2016.
7

FIGURE 9. Yampa River Basin (Maybell plus Lily) April forecast and ranked April-July unregulated inflow 

volume for years 1922-2015

FIGURE 10. Yampa River – Maybell Plus Lily April final forecast probability (percent exceedance) and 

historic April-July unregulated inflow volume for years 1963-2015
8

Probabilities of Flow Events for Spring 2016

The Flaming Gorge unregulated inflow and Yampa River forecasts are moderately dry and 

average (below median), respectively, and trending wetter. Conditions this year are wetter than 

last year and similar to 2005 and 2010. An analysis was completed to assist in the 

determination of appropriate flow objectives for spring and summer 2016. 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 may be achieved this year.

Table 1

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

Ten Similar Years to the April 1, 2016 Final Forecast

Thousand Acre-Feet (KAF)

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 825

1960 1,089

2003 1,108

1951 1,116

1925 1,121

1944 1,131

MOST 1,140

1941 1,141

1924 1,165

1948 1,188

1950 1,198

2006 1,202

MAX 1,450
9

Table 2

Spring 2016 – Days above Specific Flow Thresholds in the Yampa River 

(Maybell plus Lily)

Based on the April 1, 2016, Final 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 ten-day quantitative precipitation 

forecast (QPF). QPF is the expected amount of melted precipitation over the ten-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 4. Seasonal precipitation is 

average in the Yampa River Basin at 98 percent of normal. Precipitation was above average for 

the month of January followed by an extremely dry February. The weather pattern for March 

was active over the Yampa River basin which resulted in above average monthly precipitation

and some improvement to snowpack conditions. 

March precipitation was 130 percent of normal and the current snow water equivalent as of 

April 1 was 100 percent of median. Mean maximum temperatures were near normal for March

while mean minimum temperatures were above average. Periods of snow melt have occurred at 

lower and middle elevations.

The ridge of high pressure will bring warm and dry conditions through the end of week before a 

storm system arrives for the weekend. This the first storm in what looks to be a progressive 

weather pattern over the next week. However, this storm will be warm and any precipitation will 

be in the form of rain rather than snow. Models differ on the details for late in the weekend and 

early next week but expect to see an active weather pattern.

As a result of the warm temperatures and possible rain for the weekend, the 10-day streamflow 

forecasts are showing flows increasing over the next 10 days. The increases are a result of snow 

melt and rain.

April Final 

Forecast % Exceed

Days above 

7500 cfs

Days above 

8500 cfs

Days 

above 

9500 cfs

Days above 

10000 cfs

Days above 

12000 cfs

Days above 

14000 cfs

Days above 

15000 cfs

25% 34 26 19 17 7 3 0

50% 28 21 12 9 2 1 0

75% 23 18 5 4 0 0 0

90% 21 9 3 2 0 0 0

YAMPA
10

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 11 and 12 illustrate the probabilities of the Yampa River 

at Deerlodge river flows exceeding certain thresholds and exceedance probabilities over the 

April through July spring period. 

Table 3

Maximum Peak Daily Flow on the Yampa River at Deerlodge

Based on the April 5, 2016, RFC Analysis

Percent Exceedance (%)

FIGURE 11. RFC April 5, 2016 projection of Yampa River at Deerlodge flows exceeding thresholds for 

various percent exceedance for the 2016 April through July spring period.

CBRFC 

April 5, 2016 

Projections % Exceed

Maximum 

Daily 

Peak (cfs)

Number of 

Days to 

Peak from 

04/01/15

Number of 

Days 

Above 

10,000 cfs

Number of 

Days above 

14,000 cfs

10% 15,841 70 23 5 

25% 12,269 64 12 0

50% 10,954 56 3 0

75% 10,087 47 1 0

90% 8,743 42 0 0

YAMPA
11

FIGURE 12. RFC April 5, 2016 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. Figure 13 below 

illustrates one potential Flaming Gorge release utilizing a similar hydrologic year that 

synchronizes both the Upper Green and Yampa River Basins to the current one. 
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Table 4

Larval Trigger Study Plan Table 2 Matrix

FIGURE 13. Flaming Gorge release, Yampa and Jensen flows for WY1995 and projected operations 

under LTSP using 1995 flows. Shaded grey area indicates larval presence. 
13

FIGURE 14. Flaming Gorge release, Yampa and Jensen flows for WY2010 and projected operations 

under LTSP using 2010 flows. Shaded grey area indicates larval presence. 

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.” 
14

The potential classifications for 2016 are as follows:

Moderately Dry Classification

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

from 422,000 AF to 770,000 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 5 – 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

Dry Classification

It is likely that hydrologic conditions into Flaming Gorge Reservoir will change before 

implementation of the proposed 2016 flow objectives. In the event conditions become drier and 

the Flaming Gorge Reservoir unregulated inflow forecast for April through July falls below 

422,000 AF, the hydrological classification would be dry. 

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

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

for a period between 1 to 7 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 dry years are:
15

Table 6 –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 2 days or more (except in extremely 

dry years (≥ 98% exceedance)

Flow Recommendations and FEIS

Average (Below Median) Classification

It is likely that hydrologic conditions into Flaming Gorge Reservoir will change before 

implementation of the proposed 2016 flow objectives. In the event conditions become wetter 

and the Flaming Gorge Reservoir unregulated inflow forecast for April through July falls 

between 770,000 AF to 1,033,000 AF, the hydrological classification would be average (below 

median). 

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 7 – 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 1,033,000 AF and 1,315,000 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 
16

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 8 – 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

Flow Recommendations and FEIS