LONG-TERM SEDIMENT CONNECTIVITY IN A SMALL, EPISODICALLY AGGRADED MOUNTAINOUS CATCHMENT

Longitudinal sediment connectivity at catchment scales can be expressed by sediment delivery ratios (SDR); i.e., yield/production. Analysis of SDRs over time and space reveal complexities in connectivity due to spatial variations in topography (e.g., mountains vs. flat catchments, valley widths, etc.) and dynamics of changing land use (e.g., time elapsed since disturbance). Studies of SDRs in 20^th century North America show SDR values << 1 in small drainages due to storage in headwaters and a negative correlation with drainage area indicating poor longitudinal connectivity. Studies elsewhere (e.g., mountains with long erosion histories) show complex SDR patterns with some high values downstream.

This study examines a case where large sediment production began and ended suddenly over 31 years (1853-1884). Volumetric sediment budgets were constructed for upper Steephollow Creek, a small (54.6 km²) tributary in the Sierra Nevada, California, using high-resolution (1-m) airborne LiDAR topographic data to compute digital elevation models (DEM). DEMs for (1) pre- and post-mining at each mine, and (2) pre-mining, 1884 peak aggradation, and modern (2014) at valley bottoms were subtracted to compute DEMs of difference (DoD) and changes in volume between periods. Mine pit volumes indicate that 23.5 x 10⁶ m³ of hydraulic mining sediment (HMS) were produced in upper Steephollow basin, representing a denudation of 43.0 cm over the catchment or 1.4 mm per year. Volumes of HMS stored in valley bottoms immediately below the mines were 7.15 x 10⁶ m³ in 1884 and 3.75 x 10⁶ m³ in 2014. Thus, 30.4 % of the HMS produced remained in local storage in 1884 representing an SDR of 69.6 % (although much of this is stored nearby in lower Steephollow Creek). By 2014, storage had decreased to 16.0 % (SDR 84%). Increases in SDR with time should be expected; it is a dynamic metric for longitudinal connectivity dependent on temporal scale.

Local storage remaining in 2014 reveals processes governing long-term storage patterns. Steep tributaries were stripped of HMS between 1884 and 2014 but large HMS volumes remain sequestered behind bedrock channel cutoffs that can be recognized by LiDAR and DoD processing. Channel cutoffs store sediment, decrease lateral sediment connectivity, and ultimately decrease longitudinal and vertical connectivity.