Typically, rivers are easier to “read” than lakes. As the current flows downstream, it encounters barriers, which causes it to alter its course. These barriers can cause the current to either speed up or slow down, which creates different habitat types for fish and their prey.
On large rivers, humans may have engineered structures such as dams, wing dams, and rip-rap to alter current flow or prevent erosion.
Rivers typically flow through a hard-bottomed or restricted area that creates an area of fast water or riffle. Once through this area, the water slows to create a pool. Pools often contain deep, slow-moving water. The upstream end of the pool is referred to as the head of the pool, while the lower end of the poll is referred to as the tail, or tail-out. This process is repeated with another stretch of fast water, pool and tail-out.
As rivers meander, they create bends. The area along the outside of the bend of a river tends to have faster water and therefore is deeper; the area along the inside bend tends to be slower and shallower. In some situations, the water on the inside bend forms a current that flows in a circular manner called an eddy.
Currents within a river are not uniform from the top to the bottom. Often, the current is fastest along the top of the water and slows along the bottom. Bottom obstructions such as rocks or logs also disrupt the current and create a variety of faster and slower water speeds. Often an obstruction on the bottom of a river can be detected by a disruption of the current that is visible on the surface. The actual underwater disruption may actually be several feet upstream from where the visible evidence on the surface appears.
Rivers also have smaller streams entering that cause a mixing of currents, and islands that cause the current to split and then reform.
All of these features affect where fish will be found. Like lakes, fish are not randomly scattered throughout a river. Each fish species has its preferred habitat.
Sunfish and crappie tend to be in pools, eddies and slower-moving sections of a river. However, they can hold close to faster water, but behind, or adjacent to an obstruction that slows the water.
Abundant rain can cause flooding that temporarily changes the flow of a river. Fishing is often poor during flooding events, and can seriously affect normal fish behaviors. Droughts can reduce water flow and concentrate fish.