Flaming Gorge Technical Working 

Group 

May 5, 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 May 5, 2015 final forecast of 570,000 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 614,000 acre-feet. This forecast would fall into the dry hydrologic classification of the ROD. It 

is recommended that the official hydrologic classification for 2015 be designated at dry. 

Snow water equivalent (SWE) as of May 5, 2015, for the Upper Green River and Yampa/White 

River Basins are 53 and 50 percent of median, respectively. Flaming Gorge SWE is similar to 

2012 and 1992, while Deerlodge SWE is similar to 2002 and 2012. 

Basin Hydrology 

Green River Basin Hydrology 

The May 5, 2015, May final forecast of April through July unregulated inflow (current forecast) 

for Flaming Gorge Reservoir is 570,000 acre-feet (AF) (58% of 30-year average). This forecast 

falls at approximately 83% exceedance based on the historic unregulated inflow record (1963-

2014). 

Figure 1 illustrates the Upper Green River SWE as of April 27 and compares it against water 

years 2004 and 2012. Figure 2 shows the current forecast in relation to the historic unregulated 

inflow volumes. Figure 3 illustrates Flaming Gorge Reservoir May final forecast probability 

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

2015 YTD Snow Water Equivalent (SWE) and 1992 and 2012 percent of average SWE 

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

volume for years 1963-2014 
3 

FIGURE 3. Flaming Gorge Reservoir May 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 

614,000 AF (50% of 30-year average). This forecast falls at approximately 92% exceedance 

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

Figure 4 illustrates the Yampa River at Deerlodge Park SWE as of May 5, 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 May final 

forecast probability (percent exceedance). 
4 

FIGURE 4. Yampa River above Deerlodge SNOTEL Group. 1981-2010 percent of average SWE 

compared against 2015 YTD, and analog years 1987, 2007 and 2012 percent of median SWE 

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

inflow volume for years 1922-2014 
5 

FIGURE 6. Flaming Gorge Reservoir May 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 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 504likely 

Yampa River flows above 10,000 cfs will not be achieved this year. 
6 

Table 1 

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

Ten Similar Years to the May 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 May final Forecast 

Percent Exceedance (%) 

Colorado Basin River Forecast Center Yampa River Analysis 

Current observed streamflow on the Yampa River measured at Deerlodge is 6,140 cubic feet per second

(cfs). The Green River measured at Jensen, Utah flow is currently 6,690 cfs. The forecast for Yampa River

at Deerlodge indicates flows are expected to reach between 8,000 and 9,000 cfs on May 8‐9, Friday and

Saturday. Flows at Jensen, Utah, are expected to reach between 9,500 and 10,500 cfs on Saturday and

Sunday, May 9 and 10.  Current forecasts and the current fluctuating weather patterns indicate that

Yampa River flows will not experience another peak.  Forecast uncertainty increases beyond 5 days and

the magnitude of flows is highly uncertain.

Year

AprilJuly 

Unreg 

Inflow 

Volume 

(KAF)

MIN 504

1977 339

2002 359

2012 449

1954 555

1992 587

MOST 614

1961 635

1989 650

1994 651

1963 658

1981 659

MAX 788

May 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% 41 29 22 10 2 0 0

50% 32 24 5 2 0 0 0

75% 20 9 1 0 0 0 0

90% 4 0 0 0 0 0 0

YAMPA
7 

FIGURE 7. RFC Yampa at Deerlodge 10-day deterministic forecast. 

FIGURE 8. RFC Green River at Jensen, Utah 10-day deterministic forecast. 
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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 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. 

FIGURE 9. Flaming Gorge release, Yampa and Jensen flows for WY2002 and projected operations under 

LTSP using 2002 flows. Shaded grey area indicates larval presence. 
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FIGURE 10. Flaming Gorge release, Yampa and Jensen flows for WY2007 and projected operations 

under LTSP using 2007 flows. Shaded grey area indicates larval presence. 

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. 
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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 405,000 AF to 774,000 AF the hydrological classification would be moderately dry. 

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 3 – 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 

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

below 405,000 AF the hydrological classification would be dry. 

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

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

for a period between 1 to 7 days. These flows provide connection at Stewart Lake, Above 

Brennan and Old Charley Wash. 
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The ROD spring flow objectives for 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 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 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. 

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 
12 

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