Flaming Gorge Technical Working 

Group

March 7, 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 

1, 2017 final forecast was 2,260,000 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 1,500,000 acre-feet. This forecast would fall into the average (above median) hydrologic 

classification of the ROD. 

Snow water equivalent (SWE) as of February 14, 2017, for the Upper Green River and 

Yampa/White River Basins are 175 and 120 percent of median, respectively. Flaming Gorge 

SWE is similar to 1986 and 1997, while Deerlodge SWE is similar to 2008 and 2009. The 

Tower snotel site, used as a reference point for Yampa snowpack and runoff, currently has 38.4

inches of SWE (112 percent of median). On March 7, Tower SNOTEL measured SWE inches 

of: 

• 2008 = 33.7 in; Yampa 1,846 kaf obs spring (145%)

• 2009 = 37.5 in; Yampa 1,653 kaf obs spring (130%)

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.
2

Basin Hydrology

Green River Basin Hydrology

The March 1, 2017, ESP forecast of April through July unregulated inflow (current forecast) for 

Flaming Gorge Reservoir is 2,260,000 acre-feet (AF) (231% 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 7, 2017 and compares it against 

water years 1986 and 1997. 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 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 7, 2017.

FIGURE 3. Upper Green River Basin Water Supply Forecast as of March 7, 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
5

Yampa River Basin Hydrology

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

1,500,000 AF (121% of 30-year average). This forecast falls at approximately 30% exceedance 

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

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of March 7, 2017 and compares 

it against water years 2008 and 2009. 

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 remaining stable and the March 

1, 2017, final forecast increased from 1,377 kaf to 1,500 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 2008, 2009, 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 7, 2016.

FIGURE 8. Yampa – Deerlodge Park Water Supply Forecast as of March 7, 2016.
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FIGURE 9. Yampa River Basin (Maybell plus Lily) March forecast and ranked April-July unregulated 

inflow volume for years 1922-2016

FIGURE 10. Yampa River – Maybell Plus Lily March final 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 (above

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 20,000 cfs will not be achieved this year.

Table 1

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

Ten Similar Years to the March 1, 2017 Forecast

Thousand Acre-Feet (KAF)

Year

April-July 

Unreg 

Inflow 

Volume 

(KAF)

MIN 1,030

1938 1,439

1958 1,443

1975 1,450

2016 1,468

1947 1,479

MOST 1,500

1945 1,514

1973 1,527

1993 1,543

1979 1,562

1965 1,564

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 1, 2017 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 March 6. Water year 

precipitation is slightly above average in the Yampa River Basin at 115 percent of normal. 

Precipitation was above average for the month of January followed by near average 

precipitation in February. February precipitation was 97 percent of average. The current snow 

water equivalent as of March 1 is 115 percent of median in the Yampa River Basin.

The Yampa River basin is expected to be under a zonal flow through at least Friday. This will 

result in dry conditions and a modest warming trend. Models are currently indicating a slight 

chance for precipitation Friday into Saturday before a ridge of high pressure reestablishes 

providing more dry weather. 10 day streamflow forecasts are suggesting some increases due to 

low elevation snowmelt by days 9-10 of the forecast period. 

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. 

March 1, 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% 58 41 26 19 10 6 0

50% 53 38 23 14 7 3 0

75% 51 34 21 13 6 1 0

90% 43 31 17 10 5 0 0

YAMPA
10

Table 3

Maximum Peak Daily Flow on the Yampa River at Deerlodge

Based on the March 6, 2017, RFC Analysis

Percent Exceedance (%)

FIGURE 11. RFC March 6, 2017 projection of Yampa River at Deerlodge flows exceeding thresholds for 

various percent exceedance for the 2017 April through July spring period.

CBRFC 

March 1, 2017 

Projections % Exceed

Maximum 

Daily 

Peak (cfs)

Number of 

Days to 

Peak from 

04/01/17

Number of 

Days 

Above 

10,000 cfs

Number of 

Days above 

14,000 cfs

Number of 

Days above 

16,000 cfs

10% 22,648 73 44 24 15 

25% 16,146 66 33 11 1

50% 14,221 55 22 1 0

75% 12,067 48 10 0 0

90% 10,525 39 1 0 0

YAMPA
11

FIGURE 12. RFC March 6, 2017 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.