Backyard Streams: Morphology & Urbanization

River. Creek. Stream. Ditch. Run. Swale. There are hundreds of potential names for that flowing body of water in your back yard. If we’ve had a lot of rain lately, they’re probably not all suitable for polite company. But learning the different land forms that make up a stream channel, as well as how factors such as slope and changes upstream in the watershed affect these channel forms helps us to understand the behavior of our backyard streams.

First, let’s define some terms:

Riffle – the shallower portion of the stream, usually containing larger substrate such as gravel, cobbles or boulders, where the stream flow is more turbulent.

Pool – the deeper portion of the stream channel, usually with smaller substrate such as sand and silt.

Well - developed riffles and pools generally indicate channel stability and provide for a wide variety of aquatic habitat.

Bar – a bar is an area of deposited sediment that occurs in areas of the channel with lower velocity. Point bars generally form along the inside of a bend or meander in the stream. In stable, unimpacted streams, the elevation of the top of the point bar matches that of the floodplain.

Floodplain – the floodplain is the generally flat area next to the stream channel that conveys water during a flood. Floodplains serve many important functions including flood water and sediment storage, energy dissipation and nutrient uptake. When a stream loses access to its floodplain, these functions are lost.

Three of the main components that contribute to size, shape and location of these features within a stream channel are drainage area, slope and local soils/geology. Generally, a stream channel with a steeper slope or gradient will have smaller meanders, while a stream with a shallower slope will meander more drastically. Streams with larger drainage areas tend to have larger channels to convey the runoff being contributed form that larger area.

Changes in the landscape due to urban development, stream channel straightening and the sizing and placement of bridges and culverts can quickly and drastically change a stream channel’s alignment and the size and distribution of its in-stream features, such as pool and riffles. As illustrated in the attached diagram, increased runoff from development and increased or redirected stream velocity due to culverts or other structures in the stream channel often erode the bed of the stream, causing the stream channel to lower (this process is called stream channel incision). When the streambed is lowered relative to the height of the streambanks and floodplain, more flow (and therefore more energy) is conveyed in the channel instead of the floodplain during heavy rains. Because the incised stream cannot dissipate this energy in its now disconnected floodplain, it leads to further erosion of the streambed and also the streambanks. Streambank erosion can be further exacerbated if the streambed erodes down to a bedrock layer, since this will redirect more of the erosive energy to the streambanks. Eventually the stream channel will reach a new equilibrium at a lower elevation. However, this often only comes after a major adjustment and widening of the original stream channel to accommodate a floodplain bench at the new lower elevation.