Gain access through a private locked gate, and descend about 1 mile down to the bluff's edge. On a clear day, the sunsets are unforgettable. These results confirm land use change triggered an increase in upstream erosion and fan deposition followed by incision on short time scales.This campsite is only 45 minutes from San Francisco and provides unreal ocean views of Stinson Beach, the Farallon Islands, and incredible views of the western portion of San Francisco. Trenching and tile drainage on the uplands allowed for greater transport of water down ravines and onto fans. Incision through fan surfaces also marks post-settlement changes. These rates are a three-fold increase over generous Holocene deposition rates of 0.27 cm/yr. The presence of fly ash as an in-situ stratigraphic marker at depth was used to calculate conservative post-settlement deposition rates of 0.93 and 1.67 cm/yr using observation techniques from high-powered transmitted and reflected light microscopes as well as scanning electron microscopy, respectively.
Bulk soil samples collected at 10-, 20-, 40-, 100-, and 200-centimeter depths on the selected fans as well as samples from the incised channel were analyzed for fly ash, spherical silt-sized grains that are a byproduct of coal combustion. Ravines respond quickly to sediment and hydrology fluxes in the basin that are reflected in their alluvial fans as packages of post-settlement alluvium (PSA) and incision through fan surfaces. We take advantage of six fan deposits spread throughout the LSRB to determine the fluvial response of upland agricultural land conversion on steep first-order drainages. Establishing a post-settlement chronology is difficult in the highly erosive knickzone of the Le Sueur. Ravines, first-order channels that link low-gradient uplands with the deeply-incised channel network, experienced changes in erosion rates over time from both impacts, with the erosional history preserved in alluvial fans at the mouths of ravines where they terminate on fluvial terraces.
Onto this template of on-going incision, Euro-American land clearing and drainage of previously stable upland prairie and wetlands in the mid-1800s further increased erosion rates in the basin. Seventy meters of base-level drop at the end of the last glaciation initiated millennia of incision that continues on the LSRB today.
Ravine alluvial fans in the Le Sueur River Basin (LSRB) of south-central Minnesota record post-glacial Holocene changes and modern anthropogenic disturbances to land cover and hydrology in high-latitude watersheds.
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