Quantifying the impacts of contemporary harvesting strategies on sediment and associated nutrient transport dynamics in forested watersheds

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Forested landscapes are critical source water regions that produce the majority of drinking water supplies in Canada and the United States. Previous research demonstrates that traditional landscape activities/disturbances (forest harvesting, site preparation, road construction/use and stream crossings) can negatively impact water quality, quantity and availability. In addition to the production of forestry products, contemporary harvesting practices are also used as a primary tool to manage increasing, climate-associated wildfire risks, which can have catastrophic impacts on water quality as well as the treatability of drinking water supplies. Given increasing demands on water supplies from forested source water regions, there is an urgent need to better understand the water quality impacts of contemporary forest harvesting practices used to manage forests to decrease the impacts of potentially catastrophic disturbances (wildfire) in forested watersheds. This work provides a rare opportunity to conduct a paired catchment study to investigate impacts of various forest harvesting strategies on sediment and associated nutrient transport dynamics in Alberta’s Oldman River Basin (ORB). Two previously undisturbed, fully instrumented watersheds for which 10 years of pre-disturbance data (hydrology and water quality) have been collected will be used thereby enabling comparison of effects before and after forest harvesting. One watershed will remain undisturbed while three sub-catchments of the other will be harvested using three harvesting techniques (variable retention, strip-shelter wood and partial cut). A key impact of forest disturbance on water is the delivery of fine sediments and associated nutrients to receiving streams. In Alberta, our research demonstrates that post fire water quality impacts downstream are severe, can last for decades and increase costs of water treatment and drinking water supply. In response to recent changes in upstream water quality of the Elbow River, the City of Calgary has spent over $600 million to upgrade the Glenmore water treatment plant. To provide a comparison to wildfire as a landscape disturbance on water quality, I plan to conduct field and laboratory studies in two well-described forested catchments to characterize the source, transport and fate of sediment and associated nutrients delivered to receiving streams as a result of contemporary forest harvesting disturbances. To date, no such comparisons of contemporary harvesting practices have been conducted. Knowledge generated from this study will enable land managers to select harvesting strategies to manage wildfire risks while potentially mitigating downstream propagation of sediment and associated nutrients. While some effects of forest harvesting on water have been documented elsewhere, information from other climatic regions is not applicable to the Alberta’s eastern slopes because of geographical differences that govern impacts on water. It is actually possible that in the longer term, the impacts from resource management may be less detrimental to water values than protective policies prohibiting land uses. The outcome of these strategies must be understood to sustain water supplies. While traditional drinking water source protection policies aim to minimize anthropogenic disturbances such as forest harvesting to protect water quality, forest management policies aim to protect communities and water supplies from potentially catastrophic effects of common disturbances (wildfire) that pose significant threats to water. A novel impact of this work is that it will enable these contradictory policies to be addressed from an informed, scientific perspective.