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Lake County, Ohio - Soil & Water Conservation District

Stream Morphology

    Fluvial Geomorphology…..the study of how flowing water changes the landscape. Stream channels have some measurable features; such as width, depth, and substrate. These features are not random; they are dependent largely upon amount and frequency of flood events in that stream. In order to be consistent, many stream researchers are using the term “bankfull discharge” for. The bankfull discharge was chosen because small storms happen very frequently, but not with enough flow to shape the stream. Conversely, very large flood events change the shape and features of a stream quite drastically. However, these events do not happen frequently enough to account for much of the land-stream interface changes that we can monitor. The bankfull discharge happens on average once every year and has sufficient flow to create and or change stream features. The depth and width, and size/distribution of substrate of a stream are functions of the volume and frequency of this bankfull discharge. As stream researchers, being able to identify and measure the “bankfull width” and the “bankfull depth” and their relationship to other features of the stream allows us to make predictions on the relative stability of the channel.  The graph indicates how channel dimensions change according to watershed size.

    The volume of water in a particular stream during bankfull discharges is a function of two products: the amount of precipitation (rainfall, hail, and snow), and the land surface it falls on. The amount of water that enters the stream when one inch of precipitation falls on a completely wooded watershed is different from the amount of water that enters the stream when that same one inch of precipitation falls on a watershed that has some woods and some impervious surfaces like rooftops and driveways. Essentially, a wooded watershed provides some overland flow to the stream and then slowly releases water to the stream through the soils. Additionally, a portion of the rainfall is used by the plants and never reaches the stream. The discharge increases slowly and peaks at the bankfull discharge for a short period of time and then slowly returns to “baseflow”. Baseflow is the amount of water flowing in the stream prior and after rain events and is composed of water seeping into the stream from the ground. An urbanized watershed, complete with a storm sewer system designed to return rainwater to the stream very quickly, has very little water removed by plants and very little water soaking into the soil.  The stream now experiences very high volumes of stormwater and very low volumes of baseflow.

    What does this all mean for Lake County landowners? Perhaps you have purchased a property with a stream on it. The stream flows year round and has fish and salamanders, pools of cool water and small rapids that sound very enticing. You build your home near the scenic stream and enjoy the sounds and sights. The stream has found relative stability and has a depth and width that accommodate a bankfull discharge of approximately 35cfs for a duration of 12 hours after 2” of rainfall in a 24 hour period. Upstream of your property the land has changed from a mature wooded area to a shopping plaza. Now the 2” rain event in 24 hours releases a discharge of 50cfs and then returns to a baseflow that does not support fish and/or salamanders. Another possibility is that there is some stormwater control on the site that mandates no peak discharges greater than pre-development conditions. The stormwater pond constructed on the site has a discharge of 35cfs, but for 48 hours instead of 12 hours. So now the channel is exposed to a bankfull discharge for a duration 4x the normal. Additionally, the water was exposed to the summer sun in the pond and the water in it is now too warm for the species that used to occupy your scenic stream. The stream will become unstable due to the changes in the hydrograph. When streams become unstable the bankfull width and depth begin to adjust by eroding. The stream will continue to erode until a new stable width and depth are achieved. The process by which this happens is called the Channel Evolution Model, shown below.

    The small stream that used to be 30ft from the house is now a 6ft deep and 15ft wide gully that is eroding towards your house! In addition, there is minimal base flow for sustaining the fish and salamanders that inhabited the stream. 

    Stream researchers are constantly experimenting with possible solutions. However, the best defense against this happening to you is to maintain a natural corridor along the stream of woody vegetation. The root system from trees creates a very stable and erosion resistant area along the stream bank. Removing these trees and mowing up to the bank can lead to instability. The width of this area, often call riparian corridors, will vary depending upon the drainage area. However, a good rule of thumb would be a minimum of 15’ from the bank for small streams.