The thermal regime of rivers represents the natural variation in water temperatures for a selected period (sea- sonal, daily or diel) and watercourses. Many factors can influence the thermal regime and they can be classified using different methods. Poole and Berman [2] classified the factors influencing thermal regime in two catego- ries: internal and external factors. The external factors consider the net energy and water inputs whereas internal factors are related to the fluvial processes and river characteristics (riparian zone, surface/subsurface water inte- raction, etc.). Changes in these factors will ultimately modify water temperature variability along river reaches.
According to Caissie [3] the main drivers or factors influencing stream temperatures can be classified into: atmospheric conditions, topography, stream discharge, and riverbed thermal fluxes. This study showed that the atmospheric conditions are considered the most influential group. Atmospheric conditions are principally re- sponsible for the heat exchange process at the water surface. It includes the solar radiation, air temperature, hu- midity, wind speed as well as the type and quantity of precipitation. Topography can also influence the thermal regime of rivers and it includes factors such as latitude/longitude, riparian vegetation, geology, river aspect (orientation) and upland shading (e.g., prairie vs. mountain). Some topography factors can be influenced by hu- man activities like timber harvesting, resulting in an increase in river water temperatures, especially for small streams. Stream discharge factors are mostly related to river hydraulic conditions (e.g. surface area, water vo- lume, etc.). Some stream discharge factors are extremely important like the volume of water whereas other can be neglected like the slope or waterfalls. Streambed conditions can also influence the thermal regime depending on the size of the river and these mainly include the heat conduction at riverbed and the contribution of ground- water flow.