Are All Alpine Terraces Fluvially Produced? — A Diluvial Hypothesis for Terrace Development: Grand Valley, Colorado, USA

Fluvial terraces are abandoned floodplains situated along floors of river valleys. Terrace genesis viewed from a strictly fluvial view is bimodal: erosional (strath) and depositional (fill) processes. Field observation of many terraces in alpine regions causes one to question the above explanation and ask: is there an alternate explanation? Increasing amounts of evidence suggest that fluvial processes in many alpine areas alone cannot account for terrace development during the Cenozoic. We suggest that a series of terraces associated with the upper Colorado River basin are the result of glacial outwash floods from Grand Mesa as opposed to solely river flows.

Research during the last fifty years has suggested that the Colorado River terraces are entirely fluvial features. Unfortunately, these studies lacked extensive assessment of the internal structural morphology of the terraces as well as age controls and a link to the climatic history of the region. Our goal is to offer an alternate explanation of landform evolution in the region. Our examination of the main river terrace matrices using electrical resistivity methods identified the possible presence of boulders and cobbles as well as river gravels (high Ω-m) overlying the substrate Mancos Shale (low Ω-m). Our luminescence ages for the terraces indicate multiple stages of glacial flooding (85-74 ka, 67 ka, 63-61 ka, and 58 ka), falling between Bull Lake (MIS 6) and Pinedale glaciations (MIS 2) of Grand Mesa. The internal structures of the terraces suggest high discharges exceeding that of fluvial or seasonal flood processes. The timing of glacial flooding in the tributaries and the composition and structure of the Colorado River terraces suggest rapid deposition, followed by abandonment and incision within the past 100 ka. Thus, we suggest that these terraces, as well as those in upper tributaries formed by glacial flooding as opposed to mainly fluvial processes.

We further advocate that terraces in the majority of alpine settings, having undergone glaciation nearby and resulting glacial flooding, are diluvial in origin. Our reasoning is supported by similar conclusions from other locations in the Rocky Mountains. With additional research in other alpine settings, a wider recognition of rapid terrace formation tied to climate change and glacial flooding will ensue.