Trout Management Flows 

A Literature Review and 

Stranding Risk Modelling

Mariah Giardina, USGS GCMRC

Josh Korman, Ecometric Research

Michael Yard, USGS GCMRC (Retired) 

Scott Wright, USGS (Retired) 

Matt Kaplinski, N. Arizona Univ.

Glen Bennett, USGS GCMRC (Retired) 

U.S. Department of the Interior 

U.S. Geological Survey 
Background on Trout Management Flows (TMFs) 

• Rainbow trout abundance at Little Colorado River (LCR) inflow reach is 

periodically elevated and may impact humpback chub

• Large rainbow trout recruitment events (many young) in Glen Canyon in 2008, 

and especially 2011, led to elevated abundance near the LCR

• Mechanical removal used to reduce trout abundance at LCR 2003-2006

• Can’t keep up with immigration when rates are high

• Tribes object to taking of life, especially near LCR

• Expensive

• Is there a flow regime that can limit production of young in years when 

recruitment is expected to be high?

• Limit downstream dispersal

• Avoid high densities in Glen Canyon that can lead to collapse (e.g., 2011  2014) 
Fry Shoreline Use in Relation to Discharge
Three Potential TMF Flows
1. Flow characteristics

Literature Review to Identify Factors Influencing Stranding

• Magnitude of flow change

• Down-ramp rate

• Flow range (topography)

• Duration of high (colonization) and low (mortality) flows

• Frequency of flow fluctuations (cycles)

2. Life stage and size class

3. Channel morphology

• Substrate

• Lateral slope

• Vegetation

4. Diel cycle (night vs. day)

5. Water temperature 
Literature Review – Main Findings 

Stranding is not an established management tool for fish population control

• Stranding will kill fish and upset humans

• Stranding may only affect a small % of juvenile population and therefore not 

appreciably influence the trajectory of the population
Literature Review – Main Findings 

Fish size

• Small Fry are more vulnerable to stranding (<50mm FL)

• Glen Canyon rainbow trout = May-July

Diel Cycle (day vs. night)

• More stranding if flows decreased during daytime 

• Glen Canyon = Increased air temperature 

Channel Morphology

• Shallow lateral slope leads to more stranding

• <4% slope

• Stranding higher when habitat is complex

• Aquatic vegetation 

• Isolated pools -4 mile bar (TRGD sub-reach 1C) at ~4,000 cfs PC: M. Giardina
Predicted Changes in Wetted Area with Flow

the slough

Large areas exposed between 26-18 

kcfs

• Stranding fry over this flow range requires 

sustained high flows of ~ 26 kcfs (option a). 

• Only feasible in equalization years (e.g. 2011, 

option b)

Large areas exposed between 5-3 kcfs

• Dropping flows from normal daytime highs of 

~ 16-18 kcfs to 3 kcfs is feasible given 

constraints on release volume (option c)

Majority of shoreline in Glen Canyon is 

steep

• TMFs have to occur when young fry (< 50 

mm) are using low angle shorelines

• Not all young fry will be using habitats that 

are vulnerable to stranding
Predicted Stranding Risk as a Function of Discharge

max Q

min Q

max Q

min Q

Hydropeak min Q

Lots of water needed

very little/no extra water needed

optimal strategy

a)

c)

a)

c) Daily Minimum Discharge (kcfs)
Remaining Uncertainties

Unknowns:

• Proportion of YoY population removed per TMF cycle

• Number of TMF cycles to overcome compensatory response and achieve a substantive 

population-level effect

• Duration of low flow element of TMF

• Duration of high stable flows to maximize YOY trout colonization if implementing TMF 

option a)

Key considerations: 

• Model predicting RBT recruitment, to determine if TMF is needed, has only moderate 

predictive power

• Current rainbow trout abundance and recruitment is low, and further reductions through 

TMFs will harm fishery and possibly lead to further increases in brown trout abundance