Flaming Gorge Technical Working 

Group 

April 2, 2015 Hydrology Summary 

Heather Patno

For the purposes of discussions related to implementing the ROD in 2015, 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 2014. Based on 

these statistics and the April 2, 2015 final forecast of 650,000 acre-feet for Flaming Gorge, the 

hydrologic classification will be moderately dry (70% to 90% exceedance) for spring 2015. 

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

is 732,000 acre-feet. This forecast would fall into the moderately dry hydrologic classification of 

the ROD. 

Snow water equivalent (SWE) as of April 2, 2015, for the Upper Green River and Yampa/White 

River Basins are 75 and 67 percent of median, respectively. Flaming Gorge SWE is similar to 

2007 and 2010, while Deerlodge SWE is similar to 2002 and 2012. The Tower snotel site, used 

as a reference point for Yampa snowpack and runoff, currently has 28.9 inches of SWE (65 

percent of median). On April 2, Tower SNOTEL measured SWE inches of: 

 2002 = 28.4 in; FG 438 kaf unreg inflow; Yampa 790 kaf obs spring vol 

 2012 = 27.4 in; FG 570 unreg inflow; Yampa 449 kaf obs spring vol 

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 

snow accumulation season. 

Basin Hydrology 

Green River Basin Hydrology 

The April 2, 2015 April final forecast of April through July unregulated inflow (current forecast) 

for Flaming Gorge Reservoir is 650,000 acre-feet (AF) (66% of 30-year average). This forecast 

falls at approximately 78% exceedance based on the historic unregulated inflow record (1963-

2014). 
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Figure 1 illustrates the Upper Green River SWE as of April 2 and compares it against water 

years 2007 and 2010. Figure 2 shows the current forecast in relation to the historic unregulated 

inflow volumes. Figure 3 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 

2015 YTD Snow Water Equivalent (SWE) and 2007 and 2010 percent of average SWE 

FIGURE 2. Flaming Gorge Reservoir April final forecast and ranked historic April-July unregulated inflow 

volume for years 1963-2014 
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FIGURE 3. Flaming Gorge Reservoir April final forecast probability (percent exceedance) and historic 

April-July unregulated inflow volume for years 1963-2014 

Yampa River Basin Hydrology 

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

732,000 AF (59% of 30-year average). This forecast falls at approximately 86% exceedance 

based on a ranking of the historic record (1922-2014). 

Figure 4 illustrates the Yampa River at Deerlodge Park SWE as of April 2, 2015 and compares it 

against water years 2002 and 2012. Figure 5 below shows the current forecast in relation to 

historic flow volumes. Figure 6 illustrates the Yampa River at Maybell plus Lily April final 

forecast probability (percent exceedance). 
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FIGURE 4. Yampa River above Deerlodge SNOTEL Group. 1981-2010 percent of average SWE 

compared against 2015 YTD, and analog years 2002 and 2012 percent of median SWE 

FIGURE 5. Yampa River Basin (Maybell plus Lily) April final forecast and ranked April-July unregulated 

inflow volume for years 1922-2014 
5 

FIGURE 6. Flaming Gorge Reservoir April final forecast probability (percent exceedance) and historic 

April-July unregulated inflow volume for years 1963-2014 

Probabilities of Flow Events for Spring 2015 

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

trending drier. Conditions this year are similar to 2002, 2007 and 2012. An analysis was 

completed to assist in the determination of appropriate flow objectives for spring and summer 

2015. 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 will not be achieved this year. 
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Table 1 

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

Ten Similar Years to the April Final Forecast 

Thousand Acre-Feet (KAF) 

Table 2 

Spring 2015 – Days above Specific Flow Thresholds in the Yampa River 

(Maybell plus Lily) 

Based on the April 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 five-day quantitative precipitation 

forecast (QPF). QPF is the forecasted precipitation over the five-day climate forecast period. 

The model also incorporates the ten-day forecasted temperatures. 

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 514

1981 659

2013 676

2004 678

1966 679

1990 703

MOST 732

2007 736

1987 746

2001 790

1955 845

1959 852

MAX 1,000

April Final 

Forecast % Exceed

Days above 

3500 cfs

Days above 

4500 cfs

Days 

above 

5500 cfs

Days above 

6500 cfs

Days above 

7500 cfs

Days above 

8500 cfs

Days above 

10000 cfs

25% 50 39 26 18 9 1 0

50% 45 33 21 15 4 1 0

75% 41 23 14 9 3 0 0

90% 38 22 11 5 2 0 0

YAMPA
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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 and snow conditions as of April 2.

Seasonal precipitation is below average in the Yampa River Basin at 80 percent of normal as of

April 1. March precipitation was below normal at 75 percent of average. The addition of a dry

month in March makes three consecutive months with below average precipitation. In addition,

temperatures were once again much above normal for the month of March with many locations

breaking temperature records.

As a result of the dry and warm conditions in February and March, snow conditions are dismal at

best with a few sites at record lows. April 1 snow water equivalent is much below median at 55%

of median. The large drop in the percent median of snow water equivalent since March 1 is due

to a combination of much below average precipitation and much above average temperatures

which caused early snowmelt. As of early April, the low elevation snow has been depleted.

As a result of early snowmelt, March streamflows in the Yampa River basin were much above

average with a few sites recording flows in the top 5 of their historical period of record for the

month of March.  

An upper level trough entered the basin today bringing limited snow showers and much colder

temperatures than the previous weeks.  This trough will exit the basin this evening followed by

warmer and windy conditions for the weekend.

Temperatures over the weekend will warm up from the past two days but will not warm to the

levels we have previously seen. Another storm system is forecast to impact the basin early next

week. However, current model guidance suggests this system and another system late next week

will have limited precipitation but should keep temperatures near to slightly below normal

through the end of next week.

Short term (10 day) streamflow forecasts suggest a decrease in streamflows with forecasted

cooler temperatures and potential cloud cover. Streamflows should be similar to flows prior to

the most recent warm up that occurred near the end of the month.

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 7 and 8 illustrate the probabilities of the Yampa River at 

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

through July spring period. 
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Table 3 

Maximum Peak Daily Flow on the Yampa River at Deerlodge 

Based on the April 2, 2015, RFC Analysis 

Percent Exceedance (%) 

FIGURE 7. RFC April 2, 2015 projection of Yampa River at Deerlodge flows exceeding thresholds for 

various percent exceedance for the 2015 April through July spring period. 

CBRFC 

April 1, 2015 

Projections % Exceed

Maximum 

Daily 

Peak (cfs)

Number of 

Days to 

Peak from 

04/01/15

Number of 

Days 

Above 

8,300 cfs

Number of 

Days above 

10,000 cfs

10% 10,289 71 10 2 

25% 9,563 59 6 0

50% 8,268 52 0 0

75% 6,496 46 0 0

90% 5,691 37 0 0

YAMPA
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FIGURE 8. RFC March 4, 2015 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 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. 

Figures 9-11 below illustrate potential Flaming Gorge releases utilizing similar hydrologic years 

to the current one. 
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FIGURE 9. Flaming Gorge release, Yampa and Jensen flows for WY2001 and projected operations under 

LTSP using 2001 flows. Shaded grey area indicates larval presence. 

FIGURE 10. Flaming Gorge release, Yampa and Jensen flows for WY2010 and projected operations 

under LTSP using 2001 flows. Shaded grey area indicates larval presence. 
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FIGURE 11. Flaming Gorge release, Yampa and Jensen flows for WY2012 and projected operations 

under LTSP using 2012 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.” 

The potential classifications for 2015 are as follows: 

Moderately Dry Classification 

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

from 774,000 AF to 1,047,000 AF the hydrological classification would be average (below 

median). The Yampa River hydrology would fall within the moderately dry hydrologic 
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classification. It is recommended to operate Flaming Gorge within the moderately dry 

hydrologic classification. 

If the unregulated inflow forecast decreases 85,000 AF and falls within the range from 405,000 

AF to 774,000 AF the official hydrologic classification for Flaming Gorge would fall 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. These flows 

provide connection at Stewart Lake, Above Brennan and Old Charley Wash. 

The ROD spring flow objectives for moderately dry years are: 

Table 4 – 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 

Average (Below Median) Classification 

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

implementation of the proposed 2015 flow objectives. In the event conditions become wetter 

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

between 774,000 AF to 1,047,000 AF, and the Yampa River hydrologic classification increases 

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

Table 5 – 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 1,047,000 AF and 1,334,000 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: 

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 

Flow Recommendations and FEIS