How to Read a USGS Stream Gauge

CFS stands for cubic feet per second — the volume of water moving past a fixed point every second. One cubic foot is roughly the size of a basketball. So 200 CFS means 200 basketballs' worth of water flowing past the gauge every second.

Why does volume matter for fishing? Because it controls everything else: water clarity, current speed, where fish hold, and whether you can safely wade at all. A river at 600 CFS looks and fishes completely differently than the same river at 150 CFS.

Gauge height (also called stage) measures how deep the water is at the gauge location, in feet. It's related to CFS but not the same — a river at 3.5 feet of gauge height at one site might be at 200 CFS or 800 CFS depending on the channel geometry.

For fly fishing purposes, CFS is more useful than gauge height. CFS tells you flow volume. Gauge height tells you depth at one specific sensor. Use CFS to make your go/no-go decision. Use gauge height as a secondary check if you've fished a specific river before and know what a "normal" stage looks like.

Every river dashboard on this site pulls from the USGS Water Services API — the same source the National Weather Service uses. The USGS operates a network of roughly 8,200 stream gauges across the US. Colorado has excellent coverage on the rivers that matter for fly fishing.

Readings update every 15 minutes. The data is labeled "provisional" — meaning it hasn't gone through the USGS's formal quality review — but for day-to-day fishing decisions, it's accurate enough to trust. Major spikes get flagged and corrected after the fact, but what you see is real-time.

The fishable / marginal / unfishable badges on each river page aren't arbitrary — they're based on CFS thresholds specific to each river section. The South Platte at Deckers has different thresholds than the Arkansas at Salida because they're different rivers with different characters.

The thresholds were built from hatch calendar data, local guide knowledge, and standard Colorado fishing guidelines. They're not perfect — experienced anglers sometimes do well outside these windows — but they're a solid starting point for deciding whether to make the drive.

A river at 350 CFS and rising fast is a different situation than a river at 350 CFS and dropping. If you want to see the trend, click the USGS link on any river dashboard — it takes you directly to the USGS monitoring page for that gauge, where you can see a 7-day hydrograph (a graph of flow over time).

What to look for on the hydrograph:

• Flat line — stable flows, predictable fishing
• Rising sharply — storm runoff or snowmelt, conditions deteriorating
• Dropping steadily — post-runoff, conditions improving day by day
• Spiky pattern — dam-regulated release schedule (Blue River is the best local example)

Colorado's free-flowing rivers (South Platte, Arkansas, Colorado River) go through annual runoff every spring when the snowpack melts. Depending on the winter, this can start as early as late March or as late as May. During peak runoff, rivers that are normally 200 CFS can spike to 3,000–10,000+ CFS. They turn brown and opaque. Wading is dangerous or impossible.

The Blue River is the exception. Because it drains from Dillon Reservoir (a managed dam), flows are regulated and stay relatively consistent year-round — including through runoff season. If the other rivers are blown out in May or June, the Blue is usually still fishable.