Watershed Classification Predicts Streamflow Regime and Organic Carbon Dynamics in the Northeast Pacific Coastal Temperate Rainforest

Abstract

Watershed classification has long been a key tool in the hydrological sciences, but few studies have been extended to biogeochemistry. We developed a combined hydro‐biogeochemical classification for watersheds draining to the coastal margin of the Northeast Pacific coastal temperate rainforest (1,443,062 km2), including 2,695 small coastal rivers (SCR) and 10 large continental watersheds. We used cluster analysis to group SCR watersheds into 12 types, based on watershed properties. The most important variables for distinguishing SCR watershed types were evapotranspiration, slope, snowfall, and total precipitation. We used both streamflow and dissolved organic carbon (DOC) measurements from rivers (n = 104 and 90 watersheds respectively) to validate the classification. Watershed types corresponded with broad differences in streamflow regime, mean annual runoff, DOC seasonality, and mean DOC concentration. These links between watershed type and river conditions enabled the first region‐wide empirical characterization of river hydro‐biogeochemistry at the land‐sea margin, spanning extensive ungauged and unsampled areas. We found very high annual runoff (mean > 3,000 mm, n = 10) in three watershed types totaling 59,024 km2 and ranging from heavily glacierized mountain watersheds with high flow in summer to a rain‐fed mountain watershed type with high flow in fall‐winter. DOC hotspots (mean > 4 mg L−1, n = 14) were found in three other watershed types (48,557 km2) with perhumid rainforest climates and less‐mountainous topography. We described four patterns of DOC seasonality linked to watershed hydrology, with fall‐flushing being widespread. Hydro‐biogeochemical watershed classification may be useful for other complex regions with sparse observation networks.

Hydroclimate Drives Seasonal Riverine Export Across a Gradient of Glacierized High‐Latitude Coastal Catchments

“…Glaciers in Alaska are declining at a rapid rate, with a negative mass balance of −75 ± 11 Gt yr −1 between 1994 and 2013 (Larsen et al., 2015) leading to an increase of freshwater input to coastal environments. Accordingly, there has been a large amount of recent work to quantify the freshwater and biogeochemical fluxes within high latitude glacierized watersheds (Beamer et al., 2016; Bergstrom et al., 2021; Giesbrecht et al., 2022; Hood & Berner, 2009; Hood et al., 2020; Neal et al., 2010; Sergeant et al., 2020). Physical properties of stream freshwater along with constituent loads of dissolved organic carbon (DOC), particulate organic carbon (POC), and total suspended sediment (TSS) are important parameters to understand how nearshore organisms and ecosystems will change during the coming century (Arimitsu et al., 2016; Whitney et al., 2018).…”

Mobilization and Export of Particulate and Dissolved Solids and Organic Carbon From Contrasting Mountainous River Watersheds in California and Oregon

“…This categorization is based primarily on the size of the drainage areas of their main‐stem gages (<10,000 km 2 ; Table 1), short main channel lengths (193 and 179 km, respectively) and relatively small estuaries (24 and 26 km 2 , respectively) that account for ∼0.2% of the total catchment area. Notably, two recent papers investigating the Northeast Pacific Coastal Rainforest region classified the Umpqua and Eel watersheds as “rain continental”, a categorization that describes their pluvial water source (Giesbrecht et al., 2022) and their relative large areas with hydrologic regimes typical of interior ecosystems (Bidlack et al., 2021). However, examination of USGS gage records for rivers along the Pacific Northwest margin (Figures S1 and S2 in Supporting Information ) shows high degree of correlation between daily discharges of the main‐stem gages from the Eel and Umpqua rivers and those of other smaller mountainous rivers along the Northern California and Oregon margins.…”

Turbidity Currents Can Dictate Organic Carbon Fluxes Across River‐Fed Fjords: An Example From Bute Inlet (BC, Canada)

“…The watersheds of both rivers are supplied by meltwater from snow and glaciers within their catchment (e.g., Homathko has a 19.1% glacier cover, while Southgate has a 33.5% glacier cover) leading to sustained high flow in summer (Giesbrecht et al., 2022). An estimated 1,684 and 2,089 mm of mean annual precipitation (∼60% as snow) occurred between 1981 and 2010 (Wang et al., 2016) in the Homathko and Southgate Rivers, respectively (Giesbrecht et al., 2022). Rainforest vegetation and soils predominate below treeline on the Pacific side of the Coast Mountains (Meidinger & Pojar, 1991).…”