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How To Fly Fish

Use Stream Flow Data to Find Feeding Trout

The weather in Virginia had been horrible. Snow, snow, snow, and cold, cold, cold! Then came the sudden change of warm weather, with snow melt, and increased stream flow. But in Virginia, that doesn’t always mean high, muddy water. Rather, snow melt often leads to a modest rise in the stream flow, with continued clarity, and feeding trout.

For me, it is a two hour drive to one of my favorite streams, the Jackson River. Given  such a time commitment, I never go on a hunch. Rather, I check the golden goose for fly fishers, the USGS website, and go if the stream flow data suggests that a good day is likely in store. In this instance, there had only been a slight rise in the flow a few days ago, and had remained stable since.

When I arrived, there were no tracks or trails left by fly fishers, always a good sign. I busted through a foot of streamside snow, and found the water in perfect condition. The trout were hungry and on the feed. What I experienced, was one of the best days of trout fishing on the Jackson that I have ever experienced.

For those not aware, the USGS (United States Geologic Survey) provides online access to its water database for each state. Most major streams are monitored, grouped by watershed. The most recent 8 days of stream flow in CFS (cubic feet per second) are displayed in graphic form. The 30-year average is displayed as well. For some streams, the water level and temperature curve are similarly displayed.

I follow the streams that I most often fish several times a week. This is especially helpful when storms, heavy enough to affect stream flows, have passed through. I keep a diary, listing the flows that allow comfortable wading and good conditions for trout to feed. In small streams, a good flow is usually in the range of 100-200 CFS, while the range in larger streams is usually 1,000-2,000 CFS.

There are three major flow scenarios that I look for which help me determine whether a good day’s fishing is in store. The first is a steady flow-rate, in the range I know is readily waded and  has consistently fished well in the past. It is stream-specific, and coupled with water temperature and hatch activity, allows me to arrive at a stream likely to have actively feeding trout.

The second scenario is a rise in flow rate during a period of low flows. The rise typically moderates the water temperature, oxygenates the water, and brings foodstuffs with it. All these changes invigorate lethargic fish, which resume feeding. I usually wait for the higher level to be steady for a day or two. This allows time for any associated silt to clear.

The third scenario is a relatively abrupt rise and fall in flow. This usually occurs following a very heavy storm, and the water is typically heavily silted. Fish find it difficult to feed during the period of high flow. Thus, when the flow drops and the water begins to clear, fish start to feed heavily, making up for lost time!

USGS Stream Flow Chart

The graph above shows the changes in stream flow that occurred following a heavy storm in the Jackson River Valley. I would wait to fish until May 3 or 4, at which time the water should be clearing, although wading might be a bit difficult.

In addition to these scenarios, similar information saves me from making a needless or unproductive trip. In times of low stream flow, especially with high water temperatures, I avoid fishing, primarily to avoid stressing the struggling fish. I similarly avoid periods of very high flow or flooding, as they are dangerous and unproductive. In mountainous areas, conditions can be very different from one valley to another. Hence, I check the flows in each watershed before making plans for a fishing trip.

So, if you do not currently monitor the USGS website, I would encourage you to take a look, regularly. The information provided is very helpful to fly fishers of trout!

written by Al Simpson, May, 2020.

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