Instream wood increases riverbed temperature variability in a lowland sandy stream

Megan J. Klaar,Felicity S. Shelley,David M. Hannah,Stefan Krause

doi:10.1002/rra.3698

Abstract

AbstractThe (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.

Highlights

The promotion and restoration of natural process dynamics in rivers, in particular to mitigate anthropogenic impacts and climatic pressures, is an important component of river catchment science and management (Fryirs & Brierley, 2016; Wohl et al, 2005; Wohl, Lane, & Wilcox, 2015)
Our results show that instream wood plays an important role in promoting localized geomorphic complexity and thermal variation in the streambed
Due to the large amount of both spatial (4 depths at 10 locations) and temporal data (12 months) of streambed temperatures produced during the course of the study, all data were analysed by grouping into seasons represented as spring (March 20, 2015 – May 31, 2015), summer (June 1, 2015 – August 31, 2015), autumn (September 1, 2015 – November 30, 2015) and winter (December 1, 2015 – February 10, 2016) and at locations represented as “upstream of wood” (TLs 1&7; Figure 1), “at wood” (TLs 2 and 8), “downstream of wood” (TLs 3, 9 and 10) and “no wood influence” (TLs 4, 5 and 6)

Summary

| INTRODUCTION

The promotion and restoration of natural process dynamics in rivers, in particular to mitigate anthropogenic impacts and climatic pressures, is an important component of river catchment science and management (Fryirs & Brierley, 2016; Wohl et al, 2005; Wohl, Lane, & Wilcox, 2015). HEF usually results in cooler streambed temperatures in summer, and warmer hyporheic temperature in winter (Arrigoni et al, 2008; Caissie, 2006; Hannah, Malcolm, & Bradley, 2009; Hannah, Malcolm, Soulsby, & Youngson, 2004) This seasonal buffering provides important thermal microrefugia (Ashcroft, 2010) of particular importance to cold water species, including salmonids (Greer, Carlson, & Thompson, 2019; Malcolm et al, 2004). The majority of research on wood-induced HEF and its implications for thermal refugia in the hyporheic zone has focussed predominantly on small, upland gravel-bed rivers These rivers are typically characterized by high hydraulic conductivities, which permit wood-induced HEF to penetrate the streambed to a greater degree than finer sediment or lower conductivity sediments typical of lowland streams (Hester et al, 2009; Krause et al, 2014; Magliozzi, Grabowski, Packman, & Krause, 2018). We hypothesized that the finer sediments and more variable surface water temperatures found at our study site, typical of small lowland agricultural streams, would result in more extreme streambed temperatures, and a reduction in thermal buffering within the hyporheic zone

| METHODS

| RESULTS

Findings

| DISCUSSION