This occurs because a force, called shear stress, applied to the streambed increases with depth and slope.

At high flows, however, the pool scours and bed material deposits on the riffle. At low flows, pools are depositional features and riffles are scour features. The pool has a flat slope and is much deeper than the average depth.

Pools are located on the outside bends of meanders between riffles. Riffles are found entering and exiting meanders and control the streambed elevation. At low flows, water moves faster over riffles, which provides oxygen to the stream. The water depth is relatively shallow and the slope is steeper than the average slope of the channel. The riffle is a bed feature with gravel or larger size particles. Natural streams have sequences of riffles and pools or steps and pools that maintain channel slope and stability. The Rosgen stream classification is discussed in detail in AG-590-1, Application of the Rosgen Stream Classification in North Carolina.Ĭhannel Features Skip to Channel Features Therefore, it is critical to correctly identify bankfull stage when classifying streams and designing stream restoration measures. The Rosgen stream classification system uses bankfull stage as the basis for measuring the width/depth ratio and entrenchment ratio, two of the most important delineative criteria. In other words, each year there is about a 67 percent chance of having a bankfull streamflow event. On average, bankfull discharge occurs approximately every 1.5 years. If the stream is not entrenched, then bankfull is near the top of the bank ( Figure 2b). A stream with terraces close to the top of the banks is an incised, or entrenched stream. In this case, the top of the bank, which was formerly the floodplain, is called a terrace. If the stream has downcut due to changes in the watershed or streamside vegetation, the floodplain stage may be a small bench or scour line on the streambank ( Figure 2a). This may or may not be the top of the stream- bank. The bankfull stage, during bankfull flow is the point at which flooding occurs on the floodplain. Bankfull discharge is the flow that transports the majority of a stream’s sediment load over time and thereby forms the channel. The most important stream process in defining channel form is the bankfull discharge, which is sometimes referred to as the effective discharge, or dominant discharge. For these reasons natural resource managers increasingly use natural channel designs to restore impaired streams.īankfull Stage and Discharge Skip to Bankfull Stage and Discharge Channel features like pools, riffles, steps, and undercut banks provide diversity of habitat, oxygenation, and cover. Trees and shrubs along the banks provide a food source and regulate water temperatures. In addition to transporting water and sediment, natural streams also provide the habitat for many aquatic organisms including fish, amphibians, insects, mollusks, and plants. (Each of these characteristics will be defined in this fact sheet.) The study of these interactions and the resulting streams and rivers is called fluvial geomorphology. All of these characteristics are related to the interactions among climate, geology, topography, vegetation and land use of the watershed. Other physical characteristics shared by some stream types include pools, riffles, steps, point bars, meanders, floodplains, and terraces. Streams have left and right streambanks (looking downstream) and streambeds consisting of mixtures of bedrock, boulders, cobble, gravel, sand, or silt/clay. While streams and rivers vary greatly in size, shape, slope, and bed materials, all streams share common characteristics. The morphology, or size and shape, of the channel reflect all of these factors. Other factors which affect channel size and stream flow are land use, soil types, topography, and climate. The size and flow of a stream are directly related to its watershed area. Further downstream where baseflow is large enough to sustain stream flow throughout the year, perennial streams are formed. They dry up when groundwater levels drop.

These streams are partially supplied by groundwater rising to the surface as stream baseflow. Downstream from ephemeral channels are intermittent streams, which carry water during wet times of the year. These are referred to as ephemeral channels that only carry water during rainfall runoff. As surface runoff moves downslope, it concentrates in low areas and forms small stream channels. When rain falls in a watershed, it either runs off the land surface, infiltrates into the soil, or evaporates ( Figure 1). The land area draining to a stream or river is defined as its watershed. Streams and rivers are integral parts of the landscape that carry water and sediment from high elevations to downstream lakes, estuaries, and oceans.

Natural processes definition