Flaming Gorge Technical Working 

Group

March 5, 2018 Hydrology Summary

Heather E. Patno

For the purposes of discussions related to implementing the ROD in 2018, 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 2017. The March 

1, 2018 final forecast is 940,000 acre-feet for Flaming Gorge, which falls in the average (below 

median) hydrologic classification. Information regarding the Yampa River hydrology indicates 

that the hydrologic classification will likely be moderately dry (<70% and >90% exceedance) for 

spring 2018. 

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

is 790,000 acre-feet. This forecast would fall into the moderately dry hydrologic classification of 

the ROD. 

Snow water equivalent (SWE) as of March 5, 2018, for the Upper Green River and 

Yampa/White River Basins are 108 and 80 percent of median, respectively. Flaming Gorge 

SWE is similar to 1989 and 2012, while Deerlodge SWE is similar to 2013 and 2002. The 

Tower snotel site, used as a reference point for Yampa snowpack and runoff, currently has 27.9

inches of SWE (75 percent of median). On March 5, Tower SNOTEL measured SWE inches 

of: 

• 2002 = 22.6 in (3/1); Yampa 359 kaf obs spring (28%)

• 2013 = 26.5 in; Yampa 676 kaf obs spring (53%)

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, 2018, ESP forecast of April through July unregulated inflow (current forecast) for 

Flaming Gorge Reservoir is 940,000 acre-feet (AF) (96% of 30-year average). This forecast 

falls at 59 percent exceedance based on the historic unregulated inflow record (1963-2017). 

Figure 1 illustrates the Upper Green River SWE as of March 5, 2018 and compares it against 

water years 1989 and 2012. 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 remained fairly stable and the March 1, 2018, final forecast has increased from 870 kaf to 

940 kaf from two weeks ago. 

Historic year unregulated inflow volumes that compare with current snowpack are 1989 with 

total inflow into Flaming Gorge of 611 kaf (62 percent of average) and 2012 with total inflow into 

Flaming Gorge of 570 kaf (58 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 

2018 YTD Snow Water Equivalent (SWE) and 1989 and 2012 percent of average SWE, and 2017 

percent of median SWE.
3

FIGURE 2. Upper Green River Basin modeled SWE significant areas as of March 5, 2018.

FIGURE 3. Upper Green River Basin Water Supply Forecast as of March 5, 2018.
4

FIGURE 4. Flaming Gorge Reservoir March 1, 2018 forecast and ranked historic April-July unregulated 

inflow volume for years 1963-2017.

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

April-July unregulated inflow volume for years 1963-2017
5

Yampa River Basin Hydrology

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

790,000 AF (64% of 30-year average). This forecast falls at approximately 84% exceedance 

based on a ranking of the historic record (1922-2017). 

Figure 6 illustrates the Yampa River at Deerlodge Park SWE as of March 5, 2018 and compares 

it against water years 2002 and 2013. 

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 stable and the March 1, 2018, 

final forecast remained at 790 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 2018 YTD, and analog years 2002, 2013, and 2017 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.
6

FIGURE 7. Yampa River Basin modeled SWE significant areas as of March 5, 2018.

FIGURE 8. Yampa – Deerlodge Park Water Supply Forecast as of March 5, 2018.
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FIGURE 9. Yampa River Basin (Maybell plus Lily) March forecast and ranked April-July unregulated 

inflow volume for years 1922-2017

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

historic April-July unregulated inflow volume for years 1963-2017
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Probabilities of Flow Events for Spring 2018

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

and moderately dry, respectively, and trending drier. An analysis was completed to assist in the 

determination of appropriate flow objectives for spring and summer 2018. 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 March 1, 2018 Forecast

Thousand Acre-Feet (KAF)

Year

AprilJuly 

Unreg 

Inflow 

Volume 

(KAF)

MIN 465

2004 678

1966 679

1990 703

2007 736

1987 746

MOST 790

2001 790

1955 845

1959 852

1991 934

1953 938

MAX 1,150
9

Table 2

Spring 2018 – Days above Specific Flow Thresholds in the Yampa River 

(Maybell plus Lily)

Based on the March 1, 2018 Forecast

Percent Exceedance (%)

Colorado Basin River Forecast Center Yampa River Analysis

The Colorado Basin River Forecast Center (RFC) calculates exceedance probabilities based on 

thirty-five years of historic temperature and precipitation data (1981-2015) and current 

hydrologic conditions to provide projections of flow. The RFC provides projections based upon 

(1) strict observance to the historic dataset and (2) current hydrologic conditions including SWE, 

flow, and soil moisture and (3) incorporation of the five-day quantitative precipitation forecast 

(QPF). QPF is the expected amount of forecasted precipitation. 

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 

4. Water year precipitation is below average in the Yampa River Basin 

at 77 percent of normal. Precipitation has been below average for all 

months of the water year with the exception of February. February 

precipitation was 103 percent of average. The current snow water 

equivalent as of March 1 is 75 percent of median in the Yampa River 

Basin.

The Yampa River basin should see a warming and drying trend through

late in week. Models are currently indicating increasing chances for 

the return of a more active weather pattern early next week; however 

confidence is low on any details regarding a storm early next week.

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 of 

maximum peak flow 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, 2018 

Forecast % Exceed

Days above 

4000 cfs

Days 

above 

5000 cfs

Days 

above 

6000 cfs

Days 

above 

8000 cfs

Days 

above 

10000 cfs

Days 

above 

12000 cfs

Days 

above 

14000 cfs

25% 4 4 3 5 2 4 8 2 0 0

50% 4 0 3 1 1 9 3 0 0 0

75% 3 3 1 7 1 1 1 0 0 0

90% 3 0 1 4 8 0 0 0 0

YAMPA
10

Table 3

Maximum Peak Daily Flow on the Yampa River at Deerlodge

Based on the March 4, 2018, RFC Analysis

Percent Exceedance (%)

CBRFC 

March 4, 

2018 

Projections

% 

Exceed

Maximum 

Daily 

Peak (cfs)

Number 

of Days 

to Peak 

from 

04/01/18

Number 

of Days 

Above 

4,000 cfs

Number 

of Days 

above 

5,000 cfs

Number 

of Days 

above 

6,000 cfs

Number 

of Days 

above 

8,000 cfs

Number 

of Days 

above 

10,000 

cfs

Number 

of Days 

above 

12,000 

cfs

YAMPA

10% 13,692 51 51 43 38 32 17 6 

25% 10,334 47 47 36 27 12 1 0 

50% 8,444 40 40 23 16 2 0 0 

75% 6,799 24 24 13 2 0 0 0 

90% 5,547 10 10 2 0 0 0 0 

FIGURE 11. RFC March 4, 2018 projection of Yampa River at Deerlodge flows exceeding thresholds for 

various percent exceedance at a weekly time step for the 2018 April through July spring period.
11

FIGURE 12. RFC March 4, 2018 projection of Yampa River at Deerlodge maximum peak daily flow 

exceedance probabilities based on current hydrologic conditions and historical precipitation and 

temperature.