Influence of climate-modulated flooding on fluvial morphodynamics and stratigraphy

Contemporary climate change is expected to exacerbate river flooding in the future, but the potential impacts on alluvial landscapes remain ambiguous. There are two main sources of natural observations to bolster this understanding: modern rivers and fluvial stratigraphy. Observations from modern rivers directly connect floods and landscape evolution, but because the data span at maximum a few centuries, the temporal scope is insufficient to constrain the impact of future changes in flood frequency and magnitude. On the other hand, the stratigraphic record is synoptic in scope, but stochastic erosion and deposition render the record incomplete, and thus limit the precision of paleoenvironmental reconstructions. This dissertation sharpens interpretations of the geologic record by improving process-based interpretations of fluvial strata influenced by flooding, particularly by focusing on how changes in flooding intensity manifest in fluvial strata. Two main strategies are used. In the first, a stratigraphic study characterizes the signature of enhanced flooding during an abrupt warming episode in the geologic past, and in the second, experiments were conducted whereby a scaled fan delta was subjected to artificial floods while process responses were observed, and the resulting stratigraphy analyzed. Both the stratigraphic and experimental studies point to the importance of the balance between lateral and downstream sediment transport. It is shown that extreme flooding destabilizes channel banks, and promotes lateral sediment dispersal and floodplain reworking, whereas a total absence of flooding restricts sediment exchange across channel margins and directs sediment downstream. However, incremental increases in flooding do not always result in enhanced sediment exchange. Moderate amounts of flooding act to stabilize channels rather than destabilize them, because sediment delivered to channel margins during floods builds levees that confine flow and limit lateral sediment dispersal in favor of downstream transport. In this way, climate-modulated changes in flooding alter the spatiotemporal distribution of sediment, which is a crucial control enhancing or diminishing the taphonomic potential of paleoclimate records through time. Thus, levee-construction processes are the lynchpin governing fluvial responses to flooding, and are essential for reconstructing past environmental change as well as characterizing future threats to riparian communities