How to read a river: Riffles, runs, and pools

Inside this lesson:

Structural features
River behaviors
Biological elements
Riffles, runs, and pools
Riffles
Runs
Pools
How riffles, runs, and pools happen
Summing up

In our Intro to Fly Fishing class, the next level down from Watersheds in our Powers of 10 journey is to the levels of rivers and their features. One of the first activities we do is draw a river on the whiteboard.

It's simple river, to start. Two meandering banks, and an arrow indicating current direction. And then, we ask around for folks to call out what goes into it. As they do, we locate and place these "features" (crude icons and poor attempts at drawing).

It's not a trick question, although it can take a few minutes for everyone to warm up. Intuitively, we know a lot about what we find in rivers. There's a lot of different things—living and not—you'll find in a river.

Rivers contain:

• Banks and boundaries
• Rocks (of all sizes, from sand to boulders)
• Logs and woody debris
• Sediment and decomposed plant material
• Living plants (grasses, algae, moss, trees)
• Living creatures (insects, fish, birds, otters, sometimes humans!)
• Islands and rapids
• Eddies and currents
• Dams and other human-made constraints
• And much, much more

We could carve all these things up into three, probably:

• Structural features
• River behaviors
• Biological elements

Structural features

Some of these features are structural, meaning, they're core to how the river moves through the earth. Sand and gravel bars, islands, high banks, rip-rap, sandy beaches, and oxbows, where the river seems to double back on itself.

Some of these move around. Cover like logs and boulders move from season to season, pushed down by floods. A couple of structural features are extra-critical to fly-fishing. We pay special attention to them because fish tend to congregate around them.

Undercut banks, shelves, and ledges

Oftentimes the edge of the river is not dead up and down, or sloping up to a beach. A lot of times, banks are deceiving. This can seem like grass or solid ground, but the river's current will have hollowed them out, and they can become a haven for fish.

Overhanging banks make what we call a "lie"—a place fish hang out—in part because they're shaded and cool, in part because they're protected from overhead predators like birds, and in part because they're within striking range of any insects that fall into the water.

It's always good to approach any spot where you might be dealing with an overhang with caution, and consider there might be a large fish holding right at a safe spot at the edge of the river.

Feeder creeks

Oftentimes a river will have another, smaller river or stream coming into it. We call these feeder creeks. Incoming creeks are another great place to think about fishing, especially in the summer. Typically water in a feeder creek will be cooler, giving trout in the main river a place to hang out with more dissolved oxygen.

River behaviors

Rivers are alive, in that their current is constantly changing their structural features and topography, shifting everything from microscopic pieces of sediment to massive boulders in a flood. Rivers carve and re-carve their banks repeatedly, changing course.