Instream wood increases riverbed temperature variability in a lowland sandy stream

The (re)introduction of wood into rivers is becoming increasingly popular in river restoration and natural flood management schemes. While instream wood is known to promote geomorphic and hydraulic diversity, the impact of wood in driving surface water-streambed exchange and subsequent streambed temperatures remains under-researched, particularly in lowland rivers. We make use of the occurrence of three naturally occurring wood structures in a small, lowland sandy stream to determine how the presence of wood alters the geomorphic, hydraulic and thermal properties of the streambed. Our results show that instream wood plays an important role in promoting localized geomorphic complexity and thermal variation in the streambed. Locations within and immediately downstream of wood structures displayed the highest temperature range and daily variation. Locations upstream of wood structures were characterized by weaker daily temperature variation, while areas without wood displayed relatively stable streambed temperatures, with little diurnal fluctuation. Our study indicates that at this lowland site, instream wood increased seasonal temperature extremes (increased summer and decreased winter temperatures) at shallow depths by enhancing infiltration of warmer (summer) and colder (winter) surface water. This reduction in thermal buffering is likely to have significant implications to streambed-dwelling communities and highlights that the thermal impacts of wood reintroduction in lowland rivers should be considered prior to river restoration.