Abstract
Headwater streams account for more than 89% of global river networks and provide numerous ecosystem services that benefit downstream ecosystems and human water uses. It has been established that changes in climate have shifted the timing and magnitude of observed precipitation, which, at specific gages, have been directly linked to long-term reductions in large river discharge. However, climate impacts on ungaged headwater streams, where ecosystem function is tightly coupled to flow permanence along the river corridor, remain unknown due to the lack of data sets and ability to model and predict flow permanence. We analyzed a network of 10 gages with 38 to 69 years of records across a 5th-order river basin in the U.S. Pacific Northwest, finding increasing frequency of lower low-flow conditions across the basin. Next, we simulated river network expansion and contraction for a 65-yr period of record, revealing 24% and 9% declines in flowing and contiguous network length, respectively, during the driest months of the year. This study is the first to mechanistically simulate network expansion and contraction at the scale of a large river basin, informing if and how climate change is altering connectivity along river networks. While the heuristic model presented here yields basin-specific conclusions, this approach is generalizable and transferable to the study of other large river basins. Finally, we interpret our model results in the context of regulations based on flow permanence, demonstrating the complications of static regulatory definitions in the face of non-stationary climate.
Highlights
More than 89% of the global river network is headwaters (Downing et al, 2012; Allen et al, 2018), supporting ecosystem services and the health of downstream waters (Alexander et al, 2007; US EPA, 2015). These services are associated with the frequency with which streams have surface flow, and any declines in flow permanence will effectively disconnect larger rivers from their headwaters and their functions
Flow permanence is controlled by the dynamic interaction of geologic setting with hydrologic forcing (Costigan et al, 2016; Prancevic and Kirchner, 2019)
We investigate how timing and magnitude of discharge have shifted over a 65-yr period of record and yielded changes in flow permanence along mountain stream networks
Summary
More than 89% of the global river network is headwaters (Downing et al, 2012; Allen et al, 2018), supporting ecosystem services and the health of downstream waters (Alexander et al, 2007; US EPA, 2015). No observational studies have covered a sufficient period of record to evaluate if and how changing climate has altered flow permanence across river networks. We assess whether flow permanence in headwater streams has shifted over the past 65 years from the mid twentieth century baseline in response to observed changes in climate-driven hydrologic forcing. We selected the 5th-order Lookout Creek basin (Western Cascade Mountains, Oregon, USA) because of the extensive and long-term network of gages on low-order streams (Table S2) This basin is representative of the broader Pacific Northwest where climate change impacts on the timing and magnitude of moisture delivery to high elevation watersheds are known to cause declines in large rivers (Luce and Holden, 2009; Luce et al, 2013). This study considers the cascading impact of this change on stream discharge, and how discharge changes in headwaters may change flow permanence and connectivity in a river network
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