Skip to main content
(208) 558-7525 Fly Shop Hours: Hours change seasonally. Please call ahead.

Get Ready for the Big Flush

Jul 17, 2015 | Mike Lawson I_P_Dam.jpg

Near record heat in late June and early July really got the aquatic vegetation (macrophytes) growing in the Ranch. The hot weather also increased irrigation demand. Flows from Island Park Dam increased from around 1000 cfs when the Ranch opened on June 15 to more than 1600 cfs by the end of the month. These high flows continued until July 9 when flows were cut back due to reduced irrigation demand. Currently the flow is just over 1300 cfs which is still a little above normal for this time of year. Those of you fishing the Ranch likely haven’t noticed any decrease in the depth of the water. You can learn more about why by reading Dr. Rob Van Kirk’s latest blog on the Henry’s Fork Foundation website.

One thing that has been effecting dry fly fishing in the Ranch has been turbidity in the water. We’ve fielded a lot of questions in the shop as to why the water is off-color. The reason is a bit complicated but logical. The outflow at the Island Park Dam comes from two sources. Usually most of the water flows through the hydro facility which can handle up to 960 cfs. This facility likely provides some benefits to the river below. The water is drawn from a syphon deep at the bottom of the reservoir more than 50 yards out from the dam. Since water stratifies by temperature, with the heavier coldest water at the bottom, the temperature of the water flowing through the hydro is the coolest water in the reservoir. There is also an aeration system at the hydro which also keeps the dissolved oxygen at acceptable levels for trout. If additional storage water is required for irrigation it flows through the spillway on the west side of the dam. Over the past few years there hasn’t been a lot of water flowing through the spillway because irrigation demand has been low. That changed this year. Over the past few years it is likely that sediment and other residue has built up in the reservoir above the spillway. Water speeding through the spillway has likely started sucking this material through the spillway into the river below.

Is this material potentially harmful to the fishery in the Box Canyon and the Ranch? I posed this question to Dr. Van Kirk earlier this week. The Henry’s Fork Foundation has been carefully monitoring water quality both in the reservoir above and the river below. Most of the residue that is causing turbidity in the water is from organic material which is not harmful to fish or invertebrates. Like all reservoirs and lakes there is a buildup of decaying plant matter and other material on the bottom of the lake. Warm, clear weather also resulted in a green algae bloom that has also contributed to discoloration of the water. Over the coming week turbidity might get a bit worse before it gets better.

There is another serious matter regarding streamflow. A large flush through the Island Park Dam is scheduled for July 21 – 22. We’ll know more after Monday but as it stands now, more than 2000 cfs will be released through the Island Park Dam. Again, we don’t have a lot of details but the purpose is to send as much water as possible through the hydro facility at the Chester Dam. The turbines have not worked properly since they were installed and we’re told the flush is necessary to test them. We don’t like this at all but it doesn’t appear there are any other options. Before you get too upset at the Henry’s Fork Foundation for allowing this to happen you need to know that this was originally planned for June 21st. Can you imagine the impact that would have had? You can thank the Henry’s Fork Foundation working together with the Fremont-Madison Irrigation District for finding an alternative date. This flush is likely to disrupt fishing for a few days in the Box Canyon, Ranch and anywhere else on the Henry’s Fork for that matter.

There is usually a positive side to things. If our assessment is correct about the cause of turbidity in the water, much of the residue that has caused the river to get dirty should get blown out by the flush. The flow from the dam will likely be back to normal which will mean that most or all of the flow will pass through the hydro facility which will result in clean water.

Please check our website and Facebook page for additional information as it comes in. You can also contact the Henry’s Fork Foundation at, 208-652-3567,