Using LIDAR to model Mima mound evolution and regional energy balances in the Great Central Valley, California

Mima mounds, often associated with vernal pools, have historically been shrouded in genetic uncertainty. Nevertheless, emerging from the array of explanations proposed, a biological mechanism for mound formation has steadily gained strength. We use innovations in remote sensing and geomorphic modeling to develop a new approach to evaluate the microtopography. Using a digital elevation model created from LIDAR (light detection and ranging) data, morphometric values—average mound diameters, heights, slopes, and curvatures—were calculated across an 18 km² sector of a mound-pool region that covers an ancient river terrace near Merced, California. The terrain information was applied to a sediment transport model to estimate mound erosion and swale deposition rates. The mean net erosion rate was 38 cm kyr⁻¹, using a diffusion coefficient of 50 cm² yr⁻¹. At steady state, erosion must be balanced by a restorative upslope transport, and this estimate of erosion is comparable to observed rates of sediment mounding via pocket gopher burrowing (61 cm kyr⁻¹). These data suggest that bioturbation may play a dominant role in maintaining Mima mound terrain. LIDAR measurements were also used to develop a model that approximates the energy required for the formation of Mima mounds (shearing, pushing, and uplifting soil) and their maintenance (counteractions to erosion). This energy estimate was compared to estimates of energy available to gopher populations in the region. Our results indicate that gophers have ample energy to build typical Mima mounds in as little as 100 years, thus strongly supporting a biotic mechanism of Mima mound development and maintenance.