Sinuosity and Stream Health

Streams are dynamic systems, constantly changing and impacted by the surrounding landscape. This is especially apparent if we view the same stream under normal, baseflow conditions and then again during or after a large rainstorm. While we expect water levels to rise and lower, a certain amount of bed material (stones, rocks, sand, and soil) to move within the system, and channels to migrate over time, a stable stream will be able to do all of these things while also dissipating energy during high flows to maintain its shape and prevent accelerated erosion. 

There are a variety of stream physical habitat characteristics that function to dissipate flow energy and are critical for helping the stream respond to high flows that come from rainfall and snowmelt. In general, anything that helps slow the water will improve how the stream is affected by high flow conditions. Straight, deep streams will generally flow faster than a stream that is wider and bendy. Streams lined with smooth concrete will also allow water to flow faster than those with well-vegetated banks and a bed of cobbles and boulders. As we know from basic physics, force equals mass times acceleration, so as the water flow gets deeper and faster, the force behind the flow becomes more powerful and more likely to erode banks and damage streamside property. 

One physical feature of streams that helps streams manage high flows is sinuosity. Sinuosity is a measure of how much a stream meanders or bends. Having a high degree of sinuosity, or many curves, leads to a more stable stream that is better able to moderate high flows resulting from storms. This sinuosity also creates more diverse aquatic habitat for fish, insects, and other wildlife.

Degrees of stream sinuosity (Doumit, Jean & Awad, Souheil & Sakr, Samar. (2019). GIS-based stream system morphology analysis of Ibrahim and El Joz basins (Lebanon). 32. 35-52.)

Sinuosity and Flood Response

There are several ways that having a meandering stream can improve a stream's response to flooding. Having a high level of sinuosity increases the distance that water travels, reduces the slope of the stream, slows the flow of water, and increases the volume of water that the stream can hold. Generally, when water flows in a straight path, it picks up speed; however, if the stream channel bends, the water will flow straight into the bank, where the water is forced to stop and change directions. This sequence is happening so quickly that we see it as simply the slowing down of water as it follows the bends. This, combined with the increased flow path distance and reduced slope that comes from taking a more indirect path, means that sinuous streams will be less erosive, more stable, and less likely to experience major flooding during high flows.

Since winding streams are generally more stable, they are often able to access their floodplain better than straightened streams. While a stream easily accessing its floodplain means more flooding in some areas, it also means that there is much less destructive flooding downstream. As streams rise and leave the channel, the streamflow is spread out over a wider area and slows down. Water can be absorbed by the adjacent riparian area, meaning less water overall will be transported downstream when compared to a waterway that is straighter and confined within its banks. A little bit of flooding here and there is much healthier for the stream and less destructive to property than when extensive flooding happens in one spot, downstream, when all the water backs up. Recognizing that both the channel and its floodplain are crucial components of natural streams helps us accept that streams accessing their floodplains is a normal, beneficial function, rather than something to be prevented.

Sinuosity and Aquatic Habitat

While stream bends themselves are not necessarily a crucial habitat component for wildlife and biological communities, the impact bends have on stream flow and stability leads to higher quality aquatic habitat. 

As mentioned above, as water flows straight, it picks up speed, but it is forced to slow down when redirecting around bends. Water colliding with the bend leads to natural scouring and the creation of deep pools on the outside of bends, and the formation of shallower areas and even gravel bars near the inside of bends. As water follows the flow path, picking up speed in straighter sections and slowing down at bends, the width of the stream and depth of the water will vary as well. Accordingly, streams with high levels of sinuosity will have both pools and shallow, fast sections called riffles. Having different combinations of slow, fast, shallow, and deep flowing water will provide greater diversity of aquatic habitat, meaning that a stream has the potential to support a more diverse aquatic community of fish, wildlife, insects, and plant life.

Another benefit of a meandering, stable stream channel is reduced pollution. Sediment is the greatest pollutant in Pennsylvania streams, and it can come from both the surrounding landscape and from within the stream channel itself. Sediment pollution can degrade aquatic habitat in a variety of ways, from blanketing stream bottoms and eliminating habitat for aquatic macroinvertebrates (insects) to muddying waters that can clog the gills of some intolerant macroinvertebrates. Excess sediment also makes it more difficult for visual predators to find food and causes the stream to absorb more sunlight, which in turn warms the water and decreases dissolved oxygen.

Sinuousity can reduce sediment pollution in a stream in several ways. Since sinuous streams tend to be slower, less erosive, and more stable, they are more likely to be well vegetated, preventing accelerated soil erosion and sedimentation within the stream channel. Additionally, when high flows are able to access the floodplain, the water can further slow down, and some of the sediment and other pollutants being carried by the water will be deposited in the riparian area and removed from the channel. 

Evaluating Sinuosity

Physical characteristics of stream health are often measured using visual assessments. Some visual assessments will assess channel sinuosity directly by counting the number of bends in a section of stream or by calculating how much a stream is lengthened by the presence of bends.  Others, such as the Pennsylvania Rapid Assessment Protocol, will evaluate sinuosity indirectly by assessing how a channel has been altered by activities such as straightening, channelization, armoring, and otherwise constricting a stream channel.

Since meandering streams take up more space than straight streams do, some streams have been straightened in an attempt to control where water flows. Oftentimes, this is paired with channelizing or armoring stream banks to further restrict the flow of water within the channel. These activities are intended to keep flowing water in a designated space and with the hope of reducing flooding. However, because straight, channelized streams have shorter, more direct flow paths and are less able to access their floodplains, this actually results in more stream bank and bed erosion, worse flooding downstream, and degraded aquatic habitat.  Unsurprisingly, the straighter a stream is, whether it is assessed specifically as sinuosity or indirectly as a channel alteration, the lower it will score when evaluating stream health.

Sources and Additional Information 

Barbour, M.T., J. Gerritsen, B.D. Snyder, and J.B. Stribling. 1999. Rapid Bioassessment Protocols for Use in Streams and Wadeable Rivers: Periphyton, Benthic Macroinvertebrates and Fish, Second Edition.  EPA 841-B-99-002.  U.S. Environmental Protection Agency; Office of Water; Washington, D.C.

Minnesota Department of Natural Resources. The shape of healthy streams: Characteristics of Natural Watercourses, 2006.

Ohio Environmental Protection Agency Division of Surface Water. Methods for Assessing Habitat in Flowing Waters: Using the Qualitative Habitat Evaluation Index (QHEI), June 2006.

Pennsylvania Department of Environmental Protection Bureau of Waterways Engineering and Wetlands. Pennsylvania Riverine Condition Level 2 Rapid Assessment , September 9, 2016.

Rhodes, Edward C., Rocky Talchabhadel, and Taylor Jordan. "A Changing River: Long‐term Changes of Sinuosity and Land Cover in the Navasota River Watershed, Texas." River 3, no. 2 (May 7, 2024): 152–65.