Fluvial terrace formation along Wyoming’s Laramie Range as a response to increased late Pleistocene flood magnitudes Paul R. Hanson a, * , Joseph A. Mason b , Ronald J. Goble a a Department of Geosciences, University of Nebraska-Lincoln, 214 Bessey Hall, Lincoln, NE 68588-0340, USA b Department of Geography, University of Wisconsin-Madison, 207 Science Hall, Madison, WI 53706, USA Received 22 October 2004; received in revised form 29 August 2005; accepted 29 August 2005 Available online 8 November 2005 Abstract This study evaluates the potential climatic mechanisms involved in fluvial terrace genesis along Wyoming’s Laramie Range. We used optical dating methods to determine depositional ages for fluvial fills, and to calculate incision rates for terrace suites along two of the region’s larger rivers. Optical ages were determined for the five lowest terrace levels (T5–T1) which were deposited at ~59.6, 39.2, 26.3, 22.7, and 18.5 ka, and incision rates calculated for the two rivers were ~0.29–0.34 m/kyr over the last ~60 kyr. The formation of fluvial terraces in the central Rocky Mountains is commonly attributed to climatically induced changes in sediment input. According to most studies, relatively low incision rates existed during the colder periods of the Pleistocene due to high stream sediment loads, but terraces were formed during warmer interglacial periods when reduced sediment availability facilitated higher incision rates. However, this conceptual model cannot explain the incision records presented here, which show that the streams incised 9–10 m in two to three events during Oxygen Isotope Stage 2, but only 1–2 m during the warmer climates of the last ~ 18.5 ka. The stream power model we adapted to this setting suggests that late Quaternary streams operated under two basic states. During the colder conditions of the Pleistocene, higher flood magnitudes resulted in higher lateral erosion and incision rates. However, the lower stream discharge common to the warmer interglacial periods resulted in relatively inactive streams when both lateral erosion and incision rates were lowered. This model can explain the high incision rates during the cold conditions of Oxygen Isotope Stage 2, the occurrence of terrace fill ages near cold to warm transitions, and the apparent acceleration in incision rates toward the end of the last glacial cycle. Finally, this study suggests that terrace fill ages and incision rates are similar for the distally glaciated Laramie River, nonglaciated Sybille Creek, and the proximally glaciated streams that drain the Wind River Range. This implies that rivers responded synchronously to late Quaternary climate changes regardless of their influence by alpine glaciation. D 2005 Elsevier B.V. All rights reserved. Keywords: Fluvial features; Geochronology; Laramie mountains; Terraces 1. Introduction In the central Rocky Mountains the Quaternary has long been recognized as a period of net stream incision, but the preservation of strath terraces along most of the region’s streams indicates that incision rates varied over this time period (Reheis et al., 1991; Hancock and Anderson, 2002). A primary goal in the region has been to identify when streams incised and identify the climatic mechanisms responsible for these changes in incision rates. In theory, fluctuations in incision rates can be attributed to changes in the ratio of driving 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.08.010 * Corresponding author. Tel.: +1 402 472 2663; fax: +1 402 472 4917. E-mail address: phanson4@bigred.unl.edu (P.R. Hanson). Geomorphology 76 (2006) 12 – 25 www.elsevier.com/locate/geomorph