Flaming Gorge Technical Working 

Group

March 27, 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 March 

15, 2017 midmonth forecast was 2,180,000 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 1,200,000 acre-feet. This forecast would fall into the average (below median) hydrologic 

classification of the ROD. 

Snow water equivalent (SWE) as of March 27, 2017, for the Upper Green River and 

Yampa/White River Basins are 152 and 93 percent of median, respectively. Flaming Gorge 

SWE is similar to 1986 and 1997, while Deerlodge SWE is similar to 2005 and 2010. The 

Tower snotel site, used as a reference point for Yampa snowpack and runoff, currently has 39.1

inches of SWE (93 percent of median). On March 27, Tower SNOTEL measured SWE inches 

of: 

• 2005 = 36.3 in; Yampa 1,331 kaf obs spring (105%)

• 2010 = 32.2 in; Yampa 1,358 kaf obs spring (107%)

Basin Hydrology

Green River Basin Hydrology

The March 15, 2017, ESP forecast of April through July unregulated inflow (current forecast) for 

Flaming Gorge Reservoir is 2,180,000 acre-feet (AF) (222% 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 March 27, 2017 and compares it against 

water years 1986 and 1997. Figures 2 and 3 show the spatial extent of significant areas of 
2

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 continued to increase and the March 1, 2017, final forecast has increased from 1,986 kaf to 

2,260 kaf from two weeks ago. 

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

Figure 5 illustrates Flaming Gorge Reservoir March 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 modeled SWE significant areas as of March 27, 2017.

FIGURE 3. Upper Green River Basin Water Supply Forecast as of March 27, 2017.
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FIGURE 4. Flaming Gorge Reservoir March 1, 2017 forecast and ranked historic April-July unregulated 

inflow volume for years 1963-2016

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

April-July unregulated inflow volume for years 1963-2016
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Yampa River Basin Hydrology

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

1,200,000 AF (94% of 30-year average). This forecast falls at approximately 53% exceedance 

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

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of March 27, 2017 and 

compares it against water years 2005 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 is declining and the March 15, 

2017, final forecast decreased from 1,500 kaf to 1,200 kaf from two weeks 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 March final forecast probability (percent 

exceedance). 

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

compared against 2017 YTD, and analog years 2005, 2010, and 2016 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.
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FIGURE 7. Yampa River Basin modeled SWE significant areas as of March 27, 2016.

FIGURE 8. Yampa – Deerlodge Park Water Supply Forecast as of March 27, 2016.
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FIGURE 9. Yampa River Basin (Maybell plus Lily) March midmonth forecast and ranked April-July 

unregulated inflow volume for years 1922-2016

FIGURE 10. Yampa River – Maybell Plus Lily March midmonth forecast probability (percent exceedance) 

and historic April-July unregulated inflow volume for years 1963-2016
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Probabilities of Flow Events for Spring 2017

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

median), respectively, and trending wetter. 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 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 March 15, 2017 Forecast

Thousand Acre-Feet (KAF)

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 1,030

1944 1,131

1941 1,141

1924 1,165

1948 1,188

1950 1,198

MOST 1,200

2006 1,202

1956 1,211

1937 1,259

1969 1,291

1936 1,327

MAX 1,980
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Table 2

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

(Maybell plus Lily)

Based on the March 15, 2017 Forecast

Percent Exceedance (%)

March 15, 2017 

Forecast % Exceed

Days above 

6000 cfs

Days above 

8000 cfs

Days 

above 

10000 cfs

Days above 

12000 cfs

Days above 

14000 cfs

Days above 

16000 cfs

Days above 

18000 cfs

25% 53 32 22 7 3 0 0

50% 44 29 15 4 1 0 0

75% 41 28 10 3 0 0 0

90% 40 28 6 0 0 0 0

YAMPA