River flows directly determine where fish hold, how aggressively they feed, and whether your presentation even reaches them. CFS (cubic feet per second) and gage height tell you the volume and level of water moving through a river. A falling river at roughly 150% of its seasonal average, with 8-18 inches of visibility and water temperatures between 40°F and 68°F, creates the conditions where salmon and steelhead are most catchable. Understanding these numbers, and how to read them in real time, is the single biggest advantage you can build as an angler.
What Does CFS Mean and Why Should Anglers Care?
CFS stands for cubic feet per second. It measures the volume of water passing a fixed point on a river every second. Think of it as the river's throughput: a reading of 3,000 CFS means 3,000 cubic feet of water are rolling past the gage station each second.
Here is the critical thing most anglers get wrong: a CFS number means nothing without river-specific context. 3,000 CFS on the Cowlitz River is a pleasant, fishable flow. 3,000 CFS on the Sandy River means it is blown out and unfishable. The channel width, gradient, and streambed composition all determine what a given CFS reading actually looks like on the water.
CFS vs. Gage Height
You will see two primary measurements on any USGS water data page:
- Gage height is the water level measured in feet at the gage station. It tells you how high the water is relative to a fixed reference point.
- CFS is the volume of water flowing through. It tells you how much water is moving.
These two numbers do not always move in lockstep. The same gage height can produce different CFS readings over time because the river channel changes. Gravel shifts, logjams form, banks erode. A gage height of 5.0 feet might correspond to 2,400 CFS in January and 2,100 CFS in March on the same river after a flood event rearranges the streambed.
For fishing purposes, both numbers matter. Gage height tells you about wadability and access. CFS tells you about current speed, drift lanes, and how much water fish have to spread out in. Most experienced anglers develop a feel for their home rivers based on gage height because it directly relates to what they see at the bank, but CFS gives you the more complete picture of what the river is actually doing.
Building Your Own Reference
The best thing you can do is start logging conditions every time you fish. Write down the CFS, gage height, water temperature, and your assessment of clarity. Over a season, you will build a personal database that tells you exactly what "good" looks like on your water. A reading of 1,800 CFS on the Cowlitz with green-tinted water and 42°F temps might be your best day of the year. That data is worth more than any guide book.
How to Read a USGS Hydrograph for Fishing
The USGS National Water Information System (NWIS) provides real-time and historical streamflow data for thousands of gaging stations. Learning to read a hydrograph is a core skill for any serious river angler.
What You Are Looking At
A USGS hydrograph plots flow (CFS) or gage height over time. The X-axis is the date range, and the Y-axis is the measurement. Most stations display a blue line showing current conditions and a shaded gray area or dashed line representing the historical median for that date range.
Here is how to use it:
- Compare current flow to the historical median. If the blue line is tracking near or slightly above the median, conditions are probably normal and fishable. If it is two or three times the median, the river is likely running high and dirty.
- Look at the trend, not just the snapshot. A reading of 4,000 CFS that is falling from 8,000 is a completely different situation than 4,000 CFS that is rising from 2,500. The direction of the line matters more than where it sits at any given moment.
- Check the time scale. Zoom out to 7 or 14 days to see the full picture. A river that spiked three days ago and has been steadily dropping is telling you something very different than one that has been slowly climbing all week.
- Use the gage closest to your fishing section. Rivers can have multiple gage stations. A reading at the headwaters may not reflect conditions 40 miles downstream where tributaries add volume and where you actually plan to fish.
The Numbers That Matter
As a general framework:
- Flows within 10-20% of the historical average for that date: fish normally with standard techniques.
- Flows 50% or more above average that spiked quickly: the river is probably blown out. Wait for the drop.
- Flows at roughly 150% of average and falling: this is often the sweet spot. Enough water to push fish upstream, but clearing enough for them to see your gear.
Rising vs. Falling Rivers — When Should You Go?
This is the single most actionable piece of flow knowledge you can apply: fish a falling river, not a rising one.
Why Falling Rivers Fish Better
When a river is dropping after a rain event, several things happen simultaneously that stack conditions in your favor:
- Fish settle into defined holding water. As flows recede, the current consolidates into predictable channels, seams, and tailouts. Fish that were scattered across flooded banks and side channels funnel back into the main holding lies where you can reach them.
- Visibility improves by the hour. Sediment drops out of the water column as current speed decreases. A river that was chocolate brown at 8,000 CFS might show 12 inches of visibility at 4,500 CFS and be green-tinted and fishable at 3,000 CFS.
- Current speed becomes manageable. Your drift fishing gear actually holds bottom. Your bobber setup tracks through the slot instead of blowing through at twice the speed you need. You can present to fish instead of just throwing gear into a washing machine.
- Fish are actively feeding. After being hunkered down during peak flows, steelhead and salmon resume feeding behavior as conditions stabilize. They have been off the bite during the high water and are ready to eat.
The Freshette Effect
A freshette is a temporary rise in river flow caused by rain or snowmelt. It is not a flood; it is a pulse. And freshettes are one of the most powerful triggers for upstream fish migration.
Here is why: when rain raises tributary flows, it washes the scent of those tributaries downstream into the mainstem. Salmon and steelhead holding in the lower river or estuary detect that chemical signature, and it triggers them to push upstream. A freshette is nature's starting gun for a migration push.
The tactical takeaway: a freshette followed by a steady drop is the best possible scenario. The pulse moves fish. The falling water makes them catchable. Time your trips for 1-3 days after the rain stops and you are fishing during the window when fresh fish have moved in and conditions are becoming favorable.
Timing the Drop
How fast a river clears depends on its size and watershed:
- Small coastal rivers (Wilson, Nestucca, Sol Duc): often fishable 24-48 hours after rain stops.
- Medium rivers (Sandy, Skykomish, Cowlitz): typically 2-3 days to reach good shape.
- Large river systems (Skagit, Columbia tributaries, Deschutes): can take 3-5 days to clear and stabilize.
These are rough guidelines. Your own notes and experience on specific rivers will always be more accurate than generalizations.
How Water Temperature and Clarity Affect Fish Behavior
Flow volume is only part of the equation. Water temperature and clarity determine whether fish are active and whether they can find your presentation.
Temperature Thresholds
Water temperature governs fish metabolism, which directly controls feeding activity and willingness to chase or strike.
- Below 34°F: Largely inactive. Metabolism is at its lowest. Bites are rare and fish will not move far for a presentation.
- 38-40°F: Fish begin to show activity. Slow, deliberate presentations near their holding lies can draw strikes.
- 40-52°F: The prime zone for winter steelhead. Fish are active, willing to move to a presentation, and feeding aggressively relative to cold-water conditions.
- 52-58°F: Summer steelhead are in their peak activity range. Fish are energetic and responsive.
Salmon:
- 50-65°F: Most Pacific salmon species are comfortable and active in this range. Chinook, coho, and sockeye feed and migrate normally.
- 65-68°F: Stress begins. Fish seek thermal refugia — cold-water tributaries, deep pools, and spring-fed reaches. Feeding decreases.
- Above 68°F: Significant thermal stress. Catch-and-release mortality increases dramatically.
- 72-73°F: Migration effectively stops. Fish will hold in the deepest, coolest water they can find and will not move upstream until temperatures drop. This is a hard ceiling.
Practical application: Check water temperature before you commit to a long drive. If the mainstem is reading 71°F in August, you are better off targeting a cold-water tributary or waiting for a cool-down. If winter temps are sitting at 36°F, slow everything down and fish tight to structure.
Turbidity and Visibility
Turbidity is the measure of how much suspended sediment is in the water. For fishing purposes, you can assess it visually:
- 18+ inches of visibility: Clear water. Standard presentations, natural colors, lighter leaders. Fish can see well and may be line-shy.
- 8-18 inches of visibility: The productive zone for steelhead. Fish can see your gear but cannot inspect it too closely. This slight stain gives anglers an edge, reducing the fish's ability to detect leader and terminal tackle while still allowing them to find the bait or lure.
- Below 8 inches of visibility: Fish cannot reliably see your presentation. You are essentially fishing blind, and so are they. Wait for it to clear.
Visual color assessment:
- Green or milky-green: Fishable. Get on the water.
- Pea-soup green: Marginal. Fish tight to structure with big, bright offerings.
- Brown or tan with some visibility: Clearing. Could be fishable in hours. Worth sticking around.
- Chocolate brown: Unfishable. Go home or go to a different river.
River-Specific Flow Guides for Popular PNW Rivers
The following table provides general optimal flow ranges based on widely shared angler experience. These are starting points, not absolutes. Conditions, season, and the specific section you are fishing all matter.
| River | Optimal CFS Range | Gage Height Reference | Notes |
|---|---|---|---|
| Cowlitz (WA) | 1,800 – 7,500 CFS | — | Fishable across a wide range due to dam-controlled flows. Below 1,800 can be too low and clear for winter steelhead. |
| Sandy (OR) | 800 – 1,500 CFS | — | ~1,000 CFS is the sweet spot. Small watershed, blows out fast and clears fast. |
| Skagit (WA) | 2,000 – 9,000 CFS | — | Large river with a broad fishable window. Above 12,000 is typically blown. Best winter steelheading at 4,000-7,000. |
| Wilson (OR) | — | 4.2 – 4.8 ft | Coastal river where anglers reference gage height more than CFS. Above 6 ft is unfishable. |
| Deschutes (OR) | 3,500 – 5,500 CFS | — | Tailwater with relatively stable flows. Less affected by rain events. Fish it almost any time. |
| Hoh (WA) | 1,500 – 4,000 CFS | — | Glacial-fed with natural turbidity. Clears slower than rain-fed coastal streams. Best below 3,000 and dropping. |
| Skykomish (WA) | 2,000 – 8,000 CFS | — | Flashy watershed. Can blow out overnight and take 2-3 days to recover. Prime winter steelhead water at 3,000-5,000. |
| Yakima (WA) | 1,500 – 4,000 CFS | — | Irrigation-influenced flows. Best trout fishing at lower, stable flows. Upper river fishes differently than the canyon. |
A note on these ranges: Your best data is your own. These numbers are a consensus starting point. The angler who fishes the Sandy 30 times a year knows that 1,050 CFS with a green tint at Dodge Park is money. That level of specificity comes from time on the water, not from a table.
How to Adapt Your Technique to Current Conditions
Reading the data is only useful if you adjust what you do on the water based on what the data tells you.
High Water Techniques
When the river is running above its optimal range but still fishable (clearing, visibility 8+ inches, on the drop):
- Drift fishing with heavier weight. You need to get down in the water column quickly. Increase your lead or split shot to match current speed and maintain bottom contact.
- Side drifting. Allows you to cover water efficiently from a boat in high flows where wading access is limited or unsafe.
- Plunking. In very high or fast conditions, anchoring bait on the bottom and letting fish come to you can be more effective than trying to drift through structure you cannot read.
- Bigger presentations. High water means fish have a harder time finding your offering. Upsize your lures, baits, and tackle so fish can detect the profile from farther away.
Low and Clear Water Techniques
When flows drop below the optimal range or the river is running clear:
- Bobber dogging and float fishing. Allows precise, stealthy presentations through holding water without disturbing the pool. A jig under a float drifting naturally through a tailout is deadly in low, clear conditions.
- Lighter leaders and smaller presentations. Fish can see everything in clear water. Drop your leader diameter and downsize your offerings.
- Jigs. A marabou jig twitching under a float is one of the most effective low-water steelhead techniques in the Pacific Northwest.
- Dawn and dusk focus. In low, clear water, fish feed more aggressively during low-light periods. First light and last light are your highest-percentage windows.
Dirty Water Adjustments
When visibility is marginal (8-12 inches) and you are fishing the edge of productive conditions:
- Go big and bright, or big and dark. In stained water, contrast matters more than color matching. Chartreuse, hot pink, and orange create visibility. Solid black or dark purple create silhouette contrast.
- Add scent. When fish cannot see well, they rely more heavily on their lateral line and sense of smell. Shrimp oil, anise, or sardine wraps on your bait give fish another way to find your presentation.
- Slow your presentation down. Match current speed or drift slightly slower than the current. In dirty water, fish will not chase. Your offering needs to pass close to their face at a speed that gives them time to react.
Wading Safety at Elevated Flows
This is not optional advice. Elevated flows kill people every year in the Pacific Northwest.
- If you cannot see the bottom, do not wade deeper than your knees.
- Use a wading staff. Period.
- Felt soles or studded boots on slick rock are not negotiable.
- Fish from the bank or a boat when flows are above the normal range. There is no fish worth dying for.
- If the river came up overnight and you were not expecting it, get off the water immediately and reassess.
How DriftLine Puts River Intelligence in Your Pocket
Everything described in this article, reading hydrographs, comparing flows to historical medians, tracking the rise and fall, checking multiple rivers to find the one that is fishing best today, takes time. The USGS website was built for hydrologists, not for someone trying to figure out where to go fishing on Saturday morning.
That is the problem DriftLine solves.
DriftLine's river detail screens show you real-time flow data, gage height, and water temperature in a format built specifically for anglers. Instead of squinting at a government hydrograph, you see clean flow charts with the trend clearly visible: rising, falling, and how fast.
Multi-river monitoring lets you track every river in your rotation from a single dashboard. When you are deciding between the Cowlitz, the Sandy, and the Wilson for a weekend trip, you can compare conditions across all three in seconds rather than opening multiple browser tabs and cross-referencing USGS station numbers.
Gage data in context means you are not just looking at a raw CFS number. You are seeing how that number relates to what is actually fishable on that specific river, based on the kind of historical and experiential knowledge this article describes.
The anglers who consistently find fish are not luckier than you. They are better at reading conditions. DriftLine puts that ability in your pocket so you can make the call quickly and get on the right water at the right time.
Frequently Asked Questions
What CFS is too high to fish?
There is no universal "too high" number because CFS is river-specific. A flow that is fishable on the Skagit would be catastrophic on the Sandy. As a general rule, if current flows are more than 50% above the seasonal average and rose quickly from a rain event, the river is likely blown out. Check visibility: if you cannot see at least 8 inches into the water, conditions are not fishable regardless of what the CFS reads.
How long after rain can you fish a river?
It depends on the size of the watershed. Small coastal rivers like the Wilson or Nestucca can clear in 24-48 hours after rain stops. Medium rivers like the Sandy or Skykomish typically need 2-3 days. Large systems like the Skagit may take 3-5 days. Monitor the hydrograph for a consistent downward trend and check for color change from brown to green before committing to a trip.
Is it better to fish a rising or falling river?
Fish a falling river. When the river is dropping after a rain event, fish settle into predictable holding water, visibility improves steadily, and current speeds become manageable for effective presentations. A rising river scatters fish, increases turbidity, and makes drifts unpredictable. The best window is typically 1-3 days after rain stops, when flows are declining toward 150% of the seasonal average.
What water temperature is best for steelhead?
Winter steelhead become active above 38-40°F, with the prime feeding range between 40°F and 52°F. Summer steelhead fish best in the 52-58°F range. Below 34°F, metabolism drops too low for consistent feeding activity. If water temperatures are in the mid-30s, slow your presentations way down and fish them as close to holding structure as possible. Every degree matters at the cold end of the spectrum.
How do I tell if a river is too dirty to fish?
Use the visual test: wade in knee-deep and look down. If you can see your boots (roughly 12-18 inches of visibility), conditions are good. If you can see your feet but not detail (8-12 inches), it is marginal but fishable with big, bright or dark offerings and added scent. If you cannot see your feet at all (under 8 inches), the river is too dirty. Color is another indicator: green or milky-green is fishable, chocolate brown is not.
What does gage height mean for fishing?
Gage height measures the water level in feet at a specific USGS monitoring station, relative to a fixed reference point. It tells you how high the river is, which directly affects wadability, bank access, and which runs are fishable. Gage height and CFS do not always correlate perfectly because the river channel changes over time. Many experienced anglers prefer gage height because it directly relates to what they see at their fishing spots.
Is it safe to wade at high flows?
Exercise extreme caution. If the river is above its normal flow range, wading becomes significantly more dangerous. Increased current speed, obscured bottom structure, and floating debris all raise the risk. Do not wade deeper than your knees if you cannot see the bottom. Use a wading staff, wear studded boots, and fish from the bank or a boat whenever flows are elevated. Check the hydrograph before you leave the house so you know what to expect.
Does barometric pressure affect fishing when flows are changing?
Barometric pressure has a measurable but secondary effect compared to flow conditions. A falling barometer ahead of a storm system can increase fish activity briefly before the associated rain blows out the river. The practical application: if you see a pressure drop with rain in the forecast, fish that day before the front arrives. Once the rain hits and flows spike, flow and clarity become the dominant factors, and barometric pressure becomes largely irrelevant until the river stabilizes.
Conclusion
Every piece of advice in this article points to the same fundamental truth: understanding river flow data is the single most important skill separating anglers who consistently find fish from those who show up and hope for the best.
The river tells you everything you need to know before you ever leave the driveway. It tells you whether fish are moving, whether they can see your presentation, whether the water is the right temperature for active feeding, and whether conditions are improving or deteriorating. You just have to learn to listen.
Start simple. Pick your home river. Check the USGS data before every trip. Write down the CFS, gage height, water temperature, and your visual assessment of clarity. Note what you caught and how. Within a season, you will have a personal playbook that no guide book or internet forum can match.
The data is free. The river is talking. The only question is whether you are paying attention.