INTERPRETING CLIMATE-MODULATED PROCESSES OF TERRACE DEVELOPMENT ALONG THE COLORADO FRONT RANGE USING A LANDSCAPE EVOLUTION MODEL

Terraces flanking the Front Range of Colorado record long periods of lateral bedrock planation interspersed with short intervals of rapid stream incision over the past 1.5 Ma. Dates of terrace abandonment suggest that high, gravel-topped terraces formed during glacial intervals and terrace incision occurred only during the deepest interglacial intervals. Processes that have been suggested as potential forcing mechanisms for the repeated cycles of aggradation and incision include increased hillslope sediment production during glacial intervals, changes the magnitude and timing of rainfall intensity and peak flood distribution, and sediment flux from intermittently glaciated valleys.

In this study, we use a landscape evolution model to determine whether any of these processes, in isolation, is sufficient to explain the observed rates and patterns of terrace formation and abandonment along the Colorado piedmont. We use an idealized model catchment in which the upper half lies on resistant rock with cobble-sized sediment (representing the crystalline mountains) and the lower half lies on weaker rock with sand-sized particles (representing the adjacent sedimentary basin). Three different scenarios were imposed on the model: (1) sinusoidally varying hillslope diffusivity; (2) periodically increased storm intensity; and (3) sinusoidally varying sediment at a point in model.

Our model calculations show that varying hillslope diffusivity results in changes in regolith thickness and channel elevation in the plains, but the aggradation/incision signal is complicated by stochastic discharge variability, particularly in the model channels. Periods of increased storm intensity showed a clear signal of ~2 m of rapid channel aggradation and incision and significant dynamism of the rivers on the plains, but lacked widespread deposition. Adding a glacial point-source of sediment resulted in a clear signal of ~5 m of aggradation and incision in the plains channels that occurred gradually and in phase with the sediment forcing. None of these climatically driven mechanisms fully explain the pattern of terraces observed in the basins bounding the Front Range, suggesting that these mechanisms may be necessary for terrace formation, but are not sufficient in isolation.