Flaming Gorge Technical Working 

Group 

April 27, 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 17, 2015 midmonth forecast of 625,000 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 680,000 acre-feet. This forecast would fall into the moderately dry hydrologic classification of 

the ROD. 

Snow water equivalent (SWE) as of April 17, 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 

2012 and 1992, while Deerlodge SWE is similar to 2004 and 2007. The Tower snotel site, used 

as a reference point for Yampa snowpack and runoff, currently has 31.6 inches of SWE (64 

percent of median). On April 24, Tower SNOTEL measured SWE inches of: 

 1987 = 38.5 in; FG 982 kaf unreg inflow; Yampa 745 kaf obs spring vol 

 2007 = 32.3 in; FG 370 kaf unreg inflow; Yampa 735 kaf obs spring vol 

 2012 = 28.4 in; FG 570 kaf 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. 

Basin Hydrology 

Green River Basin Hydrology 

The April 17, 2015 April midmonth forecast of April through July unregulated inflow (current 

forecast) for Flaming Gorge Reservoir is 625,000 acre-feet (AF) (64% of 30-year average). This 
2 

forecast falls at approximately 79% 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 1992 and 2012. Figure 2 shows the current forecast in relation to the historic unregulated 

inflow volumes. Figure 3 illustrates Flaming Gorge Reservoir April midmonth 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 1992 and 2012 percent of average SWE 
3 

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

inflow volume for years 1963-2014 

FIGURE 3. Flaming Gorge Reservoir April midmonth forecast probability (percent exceedance) and 

historic April-July unregulated inflow volume for years 1963-2014 
4 

Yampa River Basin Hydrology 

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

680,000 AF (59% of 30-year average). This forecast falls at approximately 89% exceedance 

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

Figure 4 illustrates the Yampa River at Deerlodge Park SWE as of April 17, 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 midmonth 

forecast probability (percent exceedance). 

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 
5 

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

unregulated inflow volume for years 1922-2014 

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

historic April-July unregulated inflow volume for years 1963-2014 
6 

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. 

Table 1 

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

Ten Similar Years to the April Midmonth Forecast 

Thousand Acre-Feet (KAF) 

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 514

1963 658

1981 659

2013 676

2004 678

1966 679

MOST 680

1990 703

2007 736

1987 746

2001 790

1955 845

MAX 1,000
7 

Table 2 

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

(Maybell plus Lily) 

Based on the April Midmonth 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. 

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 17.

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

April 1.  

April month to date precipitation in the basin is below normal. However, a storm system is

currently impacting the basin. Temperatures for the first two weeks of April have been a mix of

near, below and slightly below normal. As a result of the precipitation and temperature so far for

April, the current snow water equivalent as of April 17 is 50% of median and has decreased as a

percent of median since the first of the month.  

A complicated early spring storm is currently over the basin. A closed low pressure system is

centered over southwest Colorado which is resulting in wrap around precipitation over the basin.

The low will slowly track northeast out of Colorado by Saturday afternoon. The unsettled pattern

continues through Sunday and Monday with a chance of more precipitation early in the week.

Model guidance for the end of next week is showing yet another storm system entering the

region however details of this system are not clear at this point.

Short term (10 day) stream flow forecasts suggest decreasing flows over the next 10 days as a

result of the cooler temperatures associated with the current progressive weather pattern.

April Midmonth 

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% 48 35 25 17 7 1 0

50% 43 33 21 13 4 1 0

75% 40 23 14 6 3 0 0

90% 38 22 11 5 2 0 0

YAMPA
8 

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. 

Table 3 

Maximum Peak Daily Flow on the Yampa River at Deerlodge 

Based on the April 17, 2015, RFC Analysis 

Percent Exceedance (%) 

FIGURE 7. RFC April 17, 2015 projection of Yampa River at Deerlodge flows exceeding thresholds for 

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

CBRFC 

April 17, 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% 10,658 58 11 2 

25% 9,247 45 3 0

50% 7,339 37 0 0

75% 6,195 32 0 0

90% 5,871 27 0 0

YAMPA
9 

FIGURE 8. RFC April 17, 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. 
10 

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. 
11 

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 405,000 AF to 774,000 AF the hydrological classification would be moderately dry. 
12 

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. 
13 

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