The rain felt cold on my arms and shoulders. My chest tightened with a shiver as my shirt became saturated. Oppressive humidity turned into a downpour along my long walk from the stream to my truck. Thankfully, I had a beach towel in the truck in case I slip and fall in a body of water, which has been known to happen.
Even after I wrung out my shirt and dried my head with the towel, I was still dripping on everything I touched. Our watersheds need rain, as stream flow is well below average throughout the Mid-Atlantic. With a strong summer thunderstorm, and the several preceding days of scattered storms, I was hoping stream flow would bounce back to healthier levels.
Looking through several stream gages that evening, I saw spikes across the hydrographs of local streams. Gages that track temperature showed increases in temperature matching the storms. Chilly rain should lower stream temperatures, right? Not so much.
Precipitation falls to the ground and infiltrates into the ground or runs off into the closest waterbody. Runoff temperature is dependent on four major factors. First are the weather conditions; air temperature and the amount of solar radiation directly impact the rain temperature.
Topographic settings including altitude, slope, latitude, vegetation, soils, and land cover influence the rate and temperature of runoff. As runoff follows gravity down to the stream, lake or wetland, each material it interacts with changes its temperature and composition. For instance, recent studies from Maryland Department of Natural Resources have indicated that surface ponds can increase temperature by as much as 0.3 degrees per pond. Shallow ponds heat quickly and subsequently heat down gradient groundwater. Impervious surfaces also heat the landscape and any water that flows over it. Research shows that watersheds with greater than 12.5% urban areas can increase stream temperatures by 2 to 7 degrees C following summer precipitation.
Once the runoff enters a stream, the discharge or water volume of the system mixes with the input. Lesser amounts of runoff entering larger bodies of water impact the temperature of the water body less. Conversely in smaller streams substantial amounts of runoff significantly alter the characteristics of the stream. The last crucial element of stream temperature is streambed interactions with groundwater and the hyporheic zone, which is the transitional area between groundwater and surface water. Groundwater is often much colder than surface flows, matching deeper soil temperatures. The more groundwater that feeds a stream, the colder the stream.
Valleys with limestone geology benefit from large, nutrient rich aquifers which pump cold water into streams that slowly drain the valleys through intricate, minute pathways. Some large impoundments keep the deepest waters cool and release colder water downstream to protect cold water fisheries. When groundwater aquifers are drained or streams are disconnected from interacting with groundwater, water temperatures are more dependent on air temperatures and runoff, making them hotter and more volatile.
This weekend I went to fish on a stream in southern Pennsylvania, early in the morning temperatures were in the lower sixties, by 1:30 PM the stream had risen to over 70 degrees Fahrenheit. Before I measured the temperature, I could sense the change in the stream. It became more still, less vibrant. I knew it was time to put the fly rod down and head home. Time for the streams and the trout to rest from the heat.