Flow spike to reduce smallmouth 

bass reproductive success, 

Green River, 2021 

Many co-authors and contributors 

Kevin R. Bestgen, Edward Kluender, Matthew Haworth, 

Koreen Zelasko, Ben Applegate, Lindsey Roberts, Christian

Smith, Katherine Lawry, Thomas Chart, Tildon Jones, Mike 

Partlow and Keena Elbin, many others 
Flow spike to reduce smallmouth bass 

reproductive success, Green River, 2021 
Bass abundance fluctuates over time 

2004 2007 2011 

Lily Park, Yampa River, 

Breton et al. 2014 
Variety of Smallmouth Bass Life 

Stages Susceptible 
Increased velocity and lower temperatures reduce 

smallmouth bass? 
Flow spike process: 

• Moderate to low flow year (70% of years) 

• Predict time to have flow spike based on temps

• Post runoff, pre-flow-spike monitoring 

• Implement 3-day flow at 4500 cfs 

• During and post-flow-spike monitoring to asses

effects 

• 2021 an extreme low flow and warm year, high

reproduction expected 

s 
Smallmouth bass flow spike, 

Green River, 2021 

LD-115 

GR-123B 

WP-115 & 123A 

150 RKM 

300 RKM 
Study Results 

• 21 June flow spike 

• Pre spike trip 19-21 June, found 19 study sites 

• Flow begins at noon 21 June, during spike trip 22-24 

June 

• Physical habitat shifts documented before and during 

flow spike 

• Biological sampling included drift nets, seines, 

electrofishing 
Results 

Flow Temperature 

~5,300 cfs 
Spawning in backwaters, 

male guards larvae over nest 
Effects varied by site: drift netting 

near bass spawning site SFS2021-09 
Effects varied by site SFS2021-10 
Results: physical habitat 

• 19 locations 

• BW physical habitat changes: > mid-column velocity 

Pre-SFS SFS 

Water velocity mid (m/s) 

1 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

Location 

+ 0.43 m/s 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15-115-2 16 17 18 19 
Results 

• BW physical habitat changes: < < BW temperature 

Pre-SFS SFS 

28 

26 

24 

22 

20 

18 

16 

14 

12 

10 

Water Temperature (C) 

- 6-15 C 

Yampa River 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15-1 15-2 16 17 18 19 

Location 
Results: Biological sampling, dispersal 

1 

49 

71 

25 

0 

0 

10 

20 

30 

40 

50 

60 

70 

80 

12:47-13:15 13:17-13:42 13:44-14:23 14:26-15:06 15:09-15:47 

# Bass 

22 June 2021 Start - Stop Time 

Flow 

connects 
Expectations of abundance: 

two comparisons 

2021? 
Preliminary electrofishing CPUE 

-23% 
Preliminary electrofishing CPUE 

-75% 
Preliminary seine CPUE 

-50% 
Preliminary seine CPUE 

-54% 
 

Otolith analysis 

~ 0.5 mm 
Expected smallmouth bass 

distribution of hatching dates 

Abundance 

Distribution of hatching dates 

Flow spike Expected 

8 

6 

4 

2 

0 

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 

Day of hatching 

Null model for no effect 
 

Conceptual observed smallmouth 

bass distribution of hatching dates 

0

2

4

6

8

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Abundance

Day of hatching

Distribution of hatching dates

Expected

Distribution of hatching dates 

Flow spike Observed Expected 

8 

6 

4

Abundance 

2 

0 

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 

Day of hatching 

Potential effects before, during, after spike 
Green River bass distribution 

of hatching dates results 

• Lodore and Whirlpool distributions of smallmouth 

hatching hatching dates showed reductions similar 

to the prior slide patterns 

• Lodore flow spike timing was about right, coming in 

the middle of the distribution 

• Whirlpool flow spike timing was later, due to earlier 

bass hatching, which will be accounted for in the 

next iteration 

• Effects on distributions after flow spike may be due 

to extendedcooler temperatures 
Bass size differences by reach, 

earlier spawning downstream 
All preliminary work: 

• Investigate age-0 catch rates 

• Complete seine samples for density 

estimation 

• Complete otolith analyses for hatch date 

distributions 

• Prospects for 2022? 
Summary 

• Physical and biological metrics indicate substantial 

flow spike effect 

• Negative effects on bass extended downstream 

further than anticpated 

• Turned a high year into a < average one 

• Alternative hypotheses being investigated 

• Conduct in average and lower flow years (70% of 

years) 
Many people to thank! 

NPS/Peter A. Williams