How to Fish Changing Flows on a Tailwater

Unfortunately, drought continues to impact the Southwest this year. Here on the San Juan River, one major advantage we have is that we’re fishing a cold-water tailwater. Even during hot summer conditions, cold water is still being released from Navajo Dam, allowing us to maintain a high level of fishing throughout an otherwise difficult drought year.

The challenge is that those releases can change quickly.

Because releases are heavily influenced by downstream irrigation demands, there are times when our guide staff gets only about 24 hours’ notice before flows change. Recently, we’ve seen a trend of increases in flow, and while those bumps can temporarily change fish behavior, they don’t necessarily make the fishing better or worse.

They just require you to adapt.

Here are a few things we’ve learned about fishing the San Juan when the flows are moving.

Remember: More Water Means More Cold Water

When they increase the release from Navajo Dam, they’re adding a larger volume of very cold water to the river.

Water leaving the dam is generally somewhere around 39–42 degrees. Even when air temperatures are pushing 100 degrees, we’ve recently measured water temperatures around 44 degrees within the first four miles below the dam.

Farther downstream, things change dramatically. Once you get eight or nine miles below the dam, we’ve seen water temperatures swing by as much as 15 degrees during the day, reaching into the mid-50s.

Understanding those temperature differences can help you build a much better fishing strategy.

Look for Warmer Water

After a significant bump in flows, don’t automatically assume the middle of the river is where you need to fish.

Shallow water along the banks can sometimes be a degree or two warmer than the main current or thalweg. That might not sound like much, but when you’re dealing with trout living in water in the low 40s, a couple degrees can make a noticeable difference in activity.

Look for:

• Shallow banks
• Soft edges
• Side channels
• Areas where you can clearly see the bottom
• Water receiving a lot of direct sunlight

Those areas can warm more quickly and give fish a reason to move and feed.

Consider Starting Later

Cold-water releases can also delay insect activity.

A degree or two of cooling may push a hatch later in the day, which means the normal summertime mentality of fishing as early as possible isn’t always the best approach immediately after a flow increase.

In some ways, we treat a significant cold-water bump more like late-winter or early-spring fishing.

Give the river some time to warm.

For some of our dry-fly trips recently, we haven’t even been starting until around 3 p.m. That gives the water most of the day to absorb some heat, allows insect activity to develop, and puts us on the river during a better feeding window.

There’s obviously a balance here. You don’t necessarily want to spend the hottest part of a 100-degree day baking in the sun either. But as our weather begins its seasonal cooling trend, starting later can become increasingly effective.

Adjust Your Depth

When you’re fishing deeper runs following a cold-water bump, don’t be afraid to get your flies down.

Cold water can make fish a little more lethargic, and instead of moving aggressively through the column, they may hold in the middle to lower third.

If you’re nymphing and not getting eats, think about depth before immediately changing flies.

At the same time, don’t ignore shallow water. Fish sitting along warmer edges can behave completely differently from trout sitting in the main current.

The river can essentially give you two different fisheries at the same time.

Flow Increases Can Change the Menu

One of the biggest benefits of increasing flows is that the extra water starts moving things around.

Higher releases can dislodge aquatic worms, annelids, scuds, and other food sources from the bottom and weed beds. Scuds in particular aren’t great swimmers. Once they’re knocked loose, they’re suddenly easy meals drifting through the system.

During recent flow increases, we’ve gone back to patterns that you might normally associate with colder seasons:

Worms. Red larva. Scuds.

Even in the middle of summer.

Rather than fishing entirely based on the calendar, pay attention to what the river is physically doing.

If the water is rising and weeds are being disturbed, there’s a good chance those food sources are getting knocked into the drift.

Don’t Forget the Streamer

For the streamer anglers, changing flows can create some really fun opportunities.

You don’t necessarily need to dredge the deepest part of the river. Try fishing lighter tips and keeping those streamers around banks, edges, softer water, and shallow structure.

A rising river creates new pieces of fishable water, and trout will often move into areas that weren’t productive at lower flows.

Experiment with your depth, change your angle, and cover water.

Adapt Instead of Fighting It

The biggest takeaway is simple:

Changing flows don’t automatically make the fishing better or worse. They make it different.

On a tailwater like the San Juan, you have to pay attention to more than the number on the gauge. Think about water temperature, sunlight, depth, insect timing, food being dislodged, and where fish can find the most comfortable water.

When they bump the river, fish the edges. Look shallow. Give the water some time to warm. Adjust your depth in the deeper runs. Break out the worms and scuds when the bottom starts getting stirred up.

And if you’re throwing streamers, keep moving, experiment, and have some fun with it.

The fish are still there.

You just have to adapt to where they went.