Interpreting Stream Flow Data for Better and Safer Fishing
by Judy Graham
Have you ever been in a Fly Shop, looked at the fishing report board and noticed they have the letters "cfs" posted, with a number by it? You may have wanted to ask the gal working there what that meant, but were too embarrassed to ask. This is extremely important information to understand when fishing during the spring runoff. During this time of year the snow is melting in the mountains while farmers downstream need water to irrigate their crops. This means is the rivers are high and can fluctuate in volume hourly. Let's see if we can make sense of how streamflow is measured and why this information is important to fly fishers.
The amount of water in a stream or river affects a number of things you may face on the stream or river on any given day while fishing: How safe it is to wade? How safe it is to drift? Or even, where the fish might be holding? So, what's it all about?
There are two terms you've probably heard a million times living in western Washington related to water levels in streams and rivers. You've probably most often heard them associated with flood warnings. The terms are stream height and streamflow. So, how does the height of water in a stream relate to the amount of water flowing?
Stream Stage or Gage Height
The U.S. Geological Survey folks start by measure the height of water in a stream. This is called stream stage, or stage or gage height. To make it simple, just picture some gal going out to a bridge and bolting a measuring rod to the bridge and reading how high the water level is below.
In a possible flooding situation you may have heard a newscaster say "Dunning Creek is expected to crest today at 12.5 feet". The 12.5 feet the newscaster is referring to is actually the stream stage. The USGS refers to this as "basically the height of the water surface, in feet, above an established datum plane where the stage is zero. The zero level is arbitrary, but is often close to the streambed". Ok, there's a start, but determining the amount of water flowing at different gage heights is a little more difficult.
As you know, river banks are irregular, but they tend to be flat on the bottom, and then have rising banks on the sides that widen out at the edge of the bank, kind of like a cross section of a bowl. So, because river banks are irregular, the relation between gage height and stream discharge, or flow, is not linear. This means is that if a stream gage height doubles, say from 8 to 16 feet, the flow can more than double. This diagram from the USGS shows it better than I can explain it. As you can see, two feet in height represents a lot more water flow than one foot does and on up to four feet.
Streamflow or discharge depends on the volume (amount) and velocity (speed) of the flow (water). It's basically the volume of water flowing past a certain point in a fixed unit of time. This is where the term cfs comes in. The USGS expresses the value in cubic feet per second. One cubic foot of water equals 7.48 gallons of water flowing each second, so to understand streamflow value think about 76 cfs being about 568 gallons of water (7.48 x 76 = 568.48) flowing by each second.
Here are two typical flow charts for the upper Yakima River. I chose the upper Yakima by Easton because it's near my home-water and the chart I use most often. I also included a typical Stage and Flow Chart from Cle Elum. The monitoring station for this chart is located about 12 miles further downstream form the first chart and happens to be after the confluence of the Cle Elum River. Notice the difference in flow just a few miles and adding another river makes in terms of volume. Also note how much a river can change in flow both day by day and hour by hour.
Where I fish, I know I can safely cross the river if the flow is no faster than 330 cfs. When it gets much higher than that, crossing is dangerous, so I fish places where I don't need to cross. As you can see from the first chart, on April 28, I could have gone fishing in the morning, crossed the river at my favorite spot, fished a few hours, and then been in big trouble trying to get back across to go home. This is what you need to understand. These rivers are controlled by dams, and if you're not aware that a dam may be increasing its flow during any given day, you could find yourself in trouble.
How can you check the stream or river you're planning to fish?
There are a number of ways to check the cfs for rivers in your area. Probably the best is your local fly shop. Another great resource is the USGS real-time stream flow database. It provides data for many of the rivers in your area with he simple click of a mouse. Access the USGS site at http://waterdata.usgs.gov/nwis/rt, click on your state, and check it out!