Changes in land use, including increased imperviousness and reduced forest cover, can have a significant effect on watershed and ecosystem health. Aside from changes in the flow regime and water quality, these land use changes also affect stream temperature. For example, a loss of shade allows more sunlight to reach the water. Changes in channel morphology from increased flow can lead to shallower waters, also resulting in more susceptibility to temperature increases from sunlight and air temperature.
These changes can have detrimental impacts on cold-water species, which have a well-defined and narrow range of tolerance to water temperature. By working to reduce or remediate sources of impairment, thermal pollution can be mitigated and the ecosystem may be able to return to its native state.
The table below outlines several factors that can lead to a rise in water temperature and possible remediation methods:
|Loss of forest cover and shading||
Increase forest height and canopy cover to cast longer and denser shadows
Remove non-native vegetation
Control livestock access to the stream
|Low summer instream flows||
Restore watershed features to increase infiltration
Promote infiltration of runoff
|Increased width-to-depth ratio of streams resulting in shallower waters||
Stabilize stream morphology to reduce incision and widening
Implement stream restoration to create deeper, wider channels
|Warm water runoff from ponds||
Convert ponds to wetlands
Remove inline pond connection
Retrofit ponds with bottom release structure to allow for cooler bottom water to reach the stream
|Heated run off during rain events||
Implement stormwater best management practices
Redirect stormwater runoff to turf or forested land cover
KCI performed a pilot study on a small watershed that was experiencing exceedances above the Use Class III 20 degrees Celsius temperature criteria for cold-water species. Using information collected from county agencies, local weather stations, and water and air data loggers set up in the streams, we were able to analyze temperature fluctuations. GIS analysis and flow monitoring data were used to identify the current conditions of the watershed. We then turned to modeling to study which impairments were causing the water temperature rises and to measure potential improvements. The model allowed us to simulate changes to the watershed including land use, forest cover, and stream geometry to measure the effect it had on the water temperature.
In the pilot study, we looked at heated runoff from upstream ponds, stream depth and riparian buffers. What we found is that our modeling showed little to no effect on the temperature from removing upstream ponds or changing the width to depth ratio of the streams. Some positive effects on the water temperature were found from increasing the riparian buffer and stream shading. The modeling allowed us to determine how much buffer and shade was needed to reach our target temperatures for the watershed.
For more information on stream temperature impairment, the different types of models available, and our pilot study, view my presentation from the 2017 National Watershed & Stormwater Conference, “Pilot Assessment of Stream Temperature for an Impaired Waterway.”