GEOMORPHIC RESPONSES TO MODERATE RAINFALL EVENTS IN THE AFTERMATH OF A MAJOR RECENT FIRE ACROSS THE VALLEY OF THE NIOBRARA NATIONAL SCENIC RIVER, NEBRASKA

During ongoing geologic mapping along the Niobrara National Scenic River (NIOB), the geomorphic response to a large fire provided an opportunity to better understand aspects of landscape response to climate change. During the relatively dry and warm 2012, a large (~30,000 acre) lightning-sparked prairie/woodland fire burned across the river valley and through the Niobrara Valley Preserve of The Nature Conservancy. Some of the most intensively burned areas were on pine-forested steep south-facing slopes below the resistant Caprock Member of the Ash Hollow Formation (Voorhies, 1987). Drought persisted through the following winter. During late May and early June of 2013, storms including thunderstorms resulted in moderate to locally heavy rainfall with daily totals of 1-2 inches recorded within and near the burn area on at least 3 days during May 19 – June 5. Though rainfall of such magnitude on the predominantly sandy soils of the area would not typically result in significant geomorphic activity, these events (perhaps related to enhanced runoff from transient hydrophobic soil properties of the burn area) produced significant erosion and deposition along ephemeral channels of the burn area crossed by the river road in the valley from about 1-4 km east of the Norden Bridge. Bottoms of major washes as wide as 35 m and incised below upland slopes showed evidence of bottom-wide flow with stage (including ephemeral aggradation) locally up to 2 m above the modern wash bottom indicated by modern root mats. Erosion during the events included small rills to 1 m depth below root mats within the wash bottoms through the midvalley slopes; and new gully erosion up to 3 m deep through the sandy terraced valley bottom land of the Niobrara River valley between the mouths of the upland washes and the modern Niobrara River. The main areas of deposition were on parts of alluvial fans between valley walls and the new river valley bottom gullies, as well as below the new gullies into areas of previous Niobrara River channel. Deposits are predominantly poorly sorted mineral sand that also includes common charcoal, ash and local pebble gravel. Exposures of the new gullies reveal older charcoal-bearing fan deposits interbedded with fluvial terrace sand of the Niobrara River, potentially indicating similar prehistoric fire-related sedimentation.