Streamwater colour in snow‐dominated headwater catchments: Natural variability and the effects of forest harvesting

Abstract

AbstractThis study documents the natural variability of true colour and the effects of forest harvesting in three snow‐dominated headwater catchments at the Upper Penticton Creek Experimental Watersheds, located in south‐central British Columbia. Although colour does not pose direct health risks, high values of colour can reduce the effectiveness of ultraviolet treatment systems, and can also indicate high concentrations of dissolved organic compounds, which can produce trihalomethanes following chlorination. The study employed a paired‐catchment design with an unlogged control (240 Creek) and pre‐ and post‐harvesting observations at two treatment catchments (241 and 242 Creeks). True colour varied seasonally, with high values during freshet followed by a general decline through summer, with some marked increases in association with autumn rain events. In the absence of harvesting effects, colour at all streams frequently exceeded the drinking water standard of 15 true colour units, especially for two catchments that contained organic soils, and there were significant positive relations between colour and the logarithm of stream discharge for all three catchments. The paired‐catchment analysis revealed significant increases in colour values once harvest reached 28% of the 241 Creek watershed and 21% of the 242 Creek watershed. For 241 Creek, the harvesting effect appeared to be independent of the colour value at 240 Creek. In contrast, the harvesting effect appeared to manifest in an increase in the slope of the relation for 242 Creek. The post‐harvest increase in colour did not appear to depend on streamflow for 241 Creek, whereas the colour response for 242 Creek had a strong positive relation with streamflow. These patterns of responses are hypothesized to reflect differences in the spatial patterns of imperfectly and poorly drained, wet soils in relation to locations of harvesting units, as well as differences in the morphology of the stream channel and riparian zone.Novelty StatementThis is the first paired‐catchment experiment to document variations in true colour and its response to forestry in snow‐dominated headwater catchments. A unique aspect of the study is the availability of a soil map showing the locations of imperfectly and poorly drained soils and organic soils. The combination of the experimental design, which included both pre‐ and post‐harvest monitoring and a control catchment, and the soils map, supported the formation of process‐based hypotheses to explain the natural seasonal and among‐catchment variability of colour and the responses to forest harvesting.