Complexities in the stream temperature regime of a small mixed-use watershed, Blacksburg, VA

Abstract

Stream temperature is a vital characteristic of stream ecosystems and has a strong control on chemical and biological processes. Water temperatures, particularly in small streams with low flows, can be affected by riparian vegetation and land cover. We designed a study using in situ temperature sensors to examine the annual thermal regime of Stroubles Creek, a small stream in Blacksburg, VA, across three land cover regions; urbanized, agricultural, and forested. During the warm sampling period, mean stream temperatures were: 17.8°C (±3.5°C) in the urban reaches; 20.0°C (±3.0°C) in the agricultural region; and 20.4°C (±3.3°C) in the forested area. Cold period daily stream temperatures were: 10.54°C (±3.1°C) in the urban reaches; 8.5°C (±4.0°C) in the agricultural region; and 7.7°C (±4.1°C) in the forested area. Linear regression analyses suggest that weekly mean stream and air temperatures have a significant linear relationship throughout the Stroubles Creek watershed, regardless of land cover or period. During the warm period, mean stream temperatures increased by 5.9°C downstream along 9km of the main stem from the headwater spring to the forested outflow as groundwater was exposed to air temperatures and environmental heat fluxes. Local cooling of stream water occurred in agricultural and forested reaches at sites with higher canopy, and possibly strong stream water–groundwater interactions. Stream temperatures decreased during the cold period by 4.5°C from headwaters to outflow, with groundwater inputs producing areas of local warming. Although the stream water–groundwater relationship of Stroubles Creek was not quantified in this study, analyses suggest that groundwater and hyporheic flow, along with riparian vegetation and canopy cover, could be controls on stream temperatures. Identifying sources of cooling for stream temperatures in the Stroubles Creek watershed, such as riparian vegetation and groundwater, could be useful for restoring the natural thermal regime, which has important implications for restoration of water quality and aquatic organism diversity in this mixed land use watershed.