Bugs in The Blue: An overview of aquatic life in the Blue River and Dillon Reservoir


Bugs in the blue: 

An Overview  of aquatic life in the middle Blue River

Blue River Fishing

There are two main food sources for trout between Dillon and Green Mountain Reservoir; juvenile fish and macro-invertebrates.  For the macroinvertebrates, a population originates from within the river system, and the other from reservoirs, and in our case, Lake Dillon.  Specifically, Mysis shrimp (Mysis diluviana) have been a foundational food source for trout in the Middle Blue, and they originate in Dillon Reservoir.

The story of these two food sources is very different and they result from competing characteristics.  Both are in low supply and I will do my best to explain why.  Let me prefice this with the following.  I am not a scientist.  This is my interpretation of information gathered in Phase 1 of the Blue River Integrated Watershed  Management Plan (BRIWMP) and Jon Ewert's Dillon Reservoir and Blue River Fishery Management Reports.  


Mysis Shrimp 

Mysis shrimp were introduced into Dillon Reservoir in 1970 to fatten up trout and salmon in the lake.  These shrimp are only available to tailwater trout when water is released from the bottom of the reservoir.  "This in turn is producing very cold temperatures which negatively impact the macroinvertebrates and brown trout populations," BRIWMP Page 22. 

A couple of current issues with the Mysis

1.  Artic Char were introduced into Dillon Reservoir from 2008 to 2015 and feed heavily on mysis.  Denver Post wrote an article in 2014 claiming the char were introduced to combat the "invasive" mysis once these shrimp migrated to the bottom of the reservoir evading trout and eating zooplankton.  Jon Ewert's Dillon Reservoir Fishery Management Report suggests the char were stocked to establish a self-sustaining fish population in Dillon and to prey on the Mysis.

2.  "When the dam is releasing the minimum flow of 50 CFS, this flow likely does not produce enough velocity to entrain mysis in to the tailwater, " Jon Ewert's Blue River Fishery Management Report page 4.  We will be around 50 cfs all winter in a drought year.

In summary, mysis shrimp populations could be decreasing due to the introudction of char and water releases are often too low to produce enough velocity to entrain mysis in to the tailwater.  Information on the current health of the mysis shrimp population is limited, and in my opinion, a missing piece to this story. 



Mysis Shrimp

This is a fancy noun for an insect we can see with the naked eye.  Some of these insects live in the water their entire lives while others are aquatic until they reach maturity.  For instance, aquatic worms never leave the river's depths, whereas the green drake molts to an adult phase during summer months to reproduce, in what we call dry-fly season.  

In 2020, 10 sites were sampled in the spring, summer and fall for macroinvertebrates as part of The Blue River Watershed Group (BRWG) and Trout Unlimited's (TU) Blue River Integrated Water Management Plan.  Results are still being compiled and BRWG has applied for 2021 grant money to continue this same study while adding two additional sites.  The science is coming and I will speak from a common citizen's/angler's perspective on this blog for now.  

The bug life in Stretch 2 (Silverthorne to Green Mountain) is lacking.  Water temperature is one of the most important variables in the bug population equation.  Different groups of bugs require different temperatures to survive and mature.  The vast majority of key bug groups in Colorado require water temps in or above the mide 40s to thrive, the exception being Midges that generally require 32 degrees or higher.  Average temperatures below the Dillon Dam spillway seldom exceed 45° F in our warmest summer months.  

Max temperature graph

 These cold temps are due to hypolimnion releases from the bottom of the reservoir.  The spike you see in the graph is due to a process called spill and fill when Denver Water releases some water from the top of the reservoir.  

 Lake Dillon Spillway

The photo above shows where the surface spill is funneled down to the tailwater. 



There is not an easy solution!

As anglers, it is easy to come to the conclusion that we see way more bugs when Denver Water releases water from the top of the dam.  However, historical data suggests these top water releases are not as positive as they may appear.  Here are a few examples..

1.  Jon Ewert's Fishery Management Report focused on fish stocking and population surveys shows 2014 is the only year CPW has measured brood trout that exceeded their hatchery weight.  This was the only year Denver Water released water from the bottom of the damn 365 days of the year. 

2.  The BRWIMP suggests that spill events create a rapid temperature change in the river, but also a large increase in stream flow.  Because of the low amount of wild reproduction of brown trout in the Middle Blue, rapid increases in stream flows can be detrimental to trout fry and therefore impact the overall population.  



We haven't figured out what the right combination of CFS and bottom vs top release is to produce the ideal conditions for bugs and trout.  We also haven't assessed the feasibility of said combination.  Bottom releases produce more mysis at a sufficient CFS so it is safe to assume the more +100 CFS days we have a year, the better.  Top releases result in the appearance of bugs immediately but a potentially negative impact on brown trout fry life cycles.  This is likely because the change in temperature and CFS is so rapid.  We must find ways to produce more gradual temperature and CFS changes that can be executed in ways that align with Denver Water's needs.


The BRWG and TU are actively seeking grant money to evaluate temperature profiles in Dillon Reservoir in 2021.  The plan is to compare these profiles to a large amount of new river temperature data gathered in Reach 2 in 2020 thanks to Tanner Banks and Trout Unlimited.  This is one of the largest gaps we must close before proposing improved Dillon Dam water release strategies. 



Ben McCormick    


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