FOREST PROCESSES AND A TILL-DOMINATED LANDSCAPE LIMIT HEADWATER EROSION IN THE POST-GLACIAL WHITE MOUNTAINS OF NEW HAMPSHIRE

Small mountainous rivers are thought to play a disproportionately large role delivering sediment to continental margins, which regulates the turnover and export of terrestrial carbon alongside other global biogeochemical cycles. Despite their global significance, smaller watersheds are less likely to be gauged and monitored compared with larger rivers. Long-term erosion and hydrologic data are thus lacking for these important systems. Here we use a nearly continuous record of annual sediment flux coupled with high-resolution discharge data collected from 1955-present at the Hubbard Brook (HB) Experimental Forest to quantify and characterize erosion in undisturbed post-glaciated headwater catchments of New England. Based on 72 measurements, mean annual erosion in the 13-hectare Reference Watershed 6 is 20 kg ha^-1 from 1964-2021, and over half of the sediment is wood, leaves, and other organic debris. We use fallout radionuclide data and observations of channel wood loading to better understand the climate, geologic, and ecologic controls on sediment production and transport on this well-characterized landscape.

Measured catchment-wide erosion rates have been declining over the last 70 years as the fraction of sediment which is organic increases. Soil ²¹⁰Pb inventories across the catchments are consistently 4000-8000 Bq m^-2, which is the amount supported by atmospheric deposition, and total annual export of ²¹⁰Pb from the catchment is <1% of the inputs. Taken together, we see evidence of limited soil erosion on this landscape except possibly very near the channel. Direct measurements and a model of channel wood loading to the HB streams support the hypothesis that increased channel woody debris is facilitating net sediment storage in headwaters and responsible for declining erosion rates measured over the last few decades. We hypothesize that channel wood and armoring by compact glacial till are responsible for limiting physical erosion and sediment transport in glaciated New England headwaters above valleys dominated by glaciofluvial deposits.