Using a post-glacial terrace sequence to better understand landscape evolution and paleohydrology in the eastern San Juan Mountains, USA

Abstract

Terraces provide a unique record of environmental change as they form due to changes in sediment supply and hydrology. In the upper Rio Grande drainage, terraces are post-glacial in age and have not been impacted by tectonics during that period. As such, these terraces are the result of changes to the fluvial system since glaciers retreated. We mapped four terrace units across two sections of the upper Rio Grande. We then examined soil profiles on terrace units, described the sedimentology, and dated the terraces where possible. Our findings indicate that the upper terrace (FT1) formed as an outwash terrace during glaciation and that incision began immediately after glaciers had melted around 12.1 ka. Two intermediate terraces likely formed during brief pauses in post-glacial incision as aggradation of the lowest terrace (FT4, 7–14 m below FT1) had already begun by 9.6 ka. Cumulic soils and fine-grained sediment on FT4, combined with radiocarbon ages, indicate that FT4 aggraded consistently in both rivers throughout the Holocene. Specifically, overbank deposits emplaced during decadal-scale flooding have allowed FT4 to aggrade consistently since 9.6 ka. The main channel appears to have been stable throughout the entire Holocene given that it is pinched between fine-grained deposits. Previous work indicates that other authors have noted similar landforms although they seem to be underreported in the literature. The timing of terrace formation differs from that in nearby headwater streams, indicating that headwater systems are more responsive to hillslope processes while larger trunk streams appear to receive sediment from upstream. Apparent differences in flooding between the two basins may indicate that basins with water managed by reservoirs may flood the FT4 surface less often, thereby highlighting the impact of water management on both fluvial hydrology and sediment storage.