Thermal heterogeneity along a braided floodplain river (Tagliamento River, northeastern Italy)

Abstract

Daily and seasonal water temperature patterns were investigated at 22 habitats in five geomorphic reaches along an Alpine-Mediterranean river. Study reaches spanned 2nd- to 7th-order river segments. Habitats included headwater streams, main and secondary channels, backwaters, and isolated pools. Multiple linear regression analyses extracted elevation and azimuth (aspect) out of eight geographical and environmental variables to explain average daily temperature patterns among habitats. Azimuth and, to a lesser degree, slope, depth, velocity, and canopy were primary determinants of diel temperature amplitude and maximum rates of diel heating and cooling. Within lowland floodplain reaches, the relative influence of groundwater and surface water varied substantially among habitats. Thermal variation among habitats was greatest in lowland floodplain reaches (nearly 15°C difference). In summer and autumn, variation between lowland floodplain aquatic habitats exceeded thermal variation observed in the main channel along the entire river corridor (120 km; 5–1100 m above sea level). Spatiotemporal variation in temperature was greatest in lower reaches owing to the interaction of water level and connectivity of isolated water bodies. Influence of groundwater and cool-water tributaries exemplified the importance of local factors (geomorphology and hydrology) superimposed on regional factors (climate and altitude) in determining large-scale thermal patterns.