Planning Your Day on the River: Using a USGS Stream Flow Gauge

As a full time guide , making the right choice on where to fish is a huge part of my day and an essential element completed before I even shake the clients hand. Water is a broad term in the fly fishing world and means a lot more to anglers than the wet stuff trout live in. In my experience, water is the biggest influencer on where I start my day and how I rig.

The break down:

• Flow (CFS)
• Tributaries
• Clarity
• Rising Trends
• Dropping Trends
• Temperature
• Current
• Type: Freestone, Tail Water, Spring Creek?

When planning a day the first thing I do is check a USGS stream gauge. Flow is measured in CFS, Cubic Feet a Second.

Understanding Flow:

Learning how to read a USGS gauge for you favorite river can help you plan your adventure or save you from crushing heart break, like when Sandra in 7th grade found your “anonymous” love song and read it to the class. I digress. The blue squiggle at the bottom represents what the river or stream is currently doing. The yellow triangles above represent what the average flow ( in cubic feet a second) is for that day based on how long the gauge has been operating. The blue squiggle ( very technical term for flow) in relation to the average (yellow triangle) is relative.

Just because the flow is above or below average doesn’t mean fishing is good or bad. In the graph above shows the flow on the Rio grande being 250cfs below the average. The key to understanding these graphs is knowing what the ideal fishing flows are for the river. 50-200 cfs on this section of the Rio Grande are prime fishing flows. This past season we had a low water year, so while we’re currently experiencing below average flows, our fishable flows are still good. CFS are not the same for all rivers or even different sections of river. While 50 cfs on the Rio Grande at the Cerro gauge is a good flow, 50 CFS on the Rio Grande at the Embudo gauge would be an environmental disaster.

The easiest way to find the ideal fishable flow for a river is to call your local fly shop, #flyshopstrong baby. If you don’t have a local fly shop the best way to understand CFS is to check the gauge before or after you fish that piece of water. Knowing exactly what the water will look like at “x” cfs will turn blue squiggles and yellow triangles into real, tangible information.

Pay attention to trends in water flow. Spikes in the blue line can indicate a surge in flow. Depending on the river and time of year it could mean snow melt, rain, a surge of water from a feeder creek or tributary. While all rivers are different , a severe spike generally indicates muddying water while a drop can indicate clearing. All rivers are different, interpreting a stream flow gauge will help you understand what the river is doing in real time.

This particular gauge is on the Rio Grande in Northern New Mexico, just south of our border with Colorado. Knowing the gauge’s physical location is also important in planning your day. Is the gauge above or below a major tributary, above irrigation, or 60 miles from where you intend to fish? Many rivers have multiple gauges. USGS organizes water into drainage basins and lists them from up stream to down stream. The lower the gauge on the list the lower it is on the river.

It can be important to know if the river or stream is a tailwater or free stone. In the first image below, the Rio Grande, is a free stone. It is not uncommon for a free stone to have changes in flow by the hour or by the day. Don’t be alarmed over all the small spikes and drops. Its what makes a freestone a freestone.

The Rio Grande is a Freestone. It is common for free stones to change in flow throughout the day.

Below:

The San Juan River is a tail water. Because a tail water’s flow is controlled by constant flow from a dam, the gauge will usually read level. This can change however if the gauge comes in below a tributary. The San Juan is a tail water. Because its flow is controlled by a dam, the gauge will read relatively level.

Some rivers have “problem tributaries”. In Northern New Mexico ours would be the Red River. A medium size rain or snow melt event will muddy its waters which will in turn stain the Rio Grande. Whenever I plan to fish the Rio Grande below the Red River, I always (almost) make sure to check and see what what the Red River is doing. This has saved me the pain and agony of driving to a spot, only to look down and realize the river looks like Willy Wonka’s chocolate river.

Learning how to read a USGS gauge and identify “problem tributaries” will save you time and headache all from the comfort of your couch. Check the flow before you go. You’ll be happy that you did.