NATURAL OR ANTHROPOGENIC: (MIS?)INTERPRETTING FLOODPLAIN GRAIN-SIZE STRUCTURE, FLOOD RECURRENCE AND SEDIMENTATION RATES

Everybody agrees that human activities cause landscape change and soil erosion, but the methods we use to document the net erosion rates today and in the past can be indirect and subject to large spatial and temporal biases. Unraveling the actual impacts to the environment remains a challenge. Floodplain sediment accumulation measurements are taken at discreet cores or bank exposures, and this introduces both temporal and spatial bias. Vertical accumulation rates are calculated by dividing thickness of sediment by the time-span for discrete packages of sediment. Thus, time integrates from the present to a past datum provided by ¹⁴C measurements for buried organics. Flooding episodes are often identified by the quantity of ¹⁴C dates for a specific period.

A new global compilation of Holocene floodplain accumulation rates measured over the last ~100 years are faster than those averaged over ~1000 years, which in turn are faster than those for the last ~10000 years. I argue that the pattern of rate increase in sedimentation over time is related to infilling behavior of all floodplains and not specifically tied to the supply of (anthropogenic) sediment. The apparent acceleration in sedimentation rates appears globally synchronous, despite diachronous land use and can pre-date significant human occupation.

A second observation of floodplain structure shows that the characteristic process of floodplain sediment accumulation is a periodic alternation of cut and fill. As the meandering river migrates, the channel cuts older deposits at its outer banks. The channel-fill cycle begins with fast, coarse-grain deposition at the channel bed and river bars and ends with more recent, over-bank deposition of fine-grained deposits over the whole area of rare flooding. Carbon enriches the top of the floodplain and remains stored within the floodplain. I present a model where the residence time of varying-sized particles and carbon can explain the grain-size structure and ¹⁴C date frequency in floodplains without a need for increased anthropogenically-derived fine-sediment or enhanced flooding episodes.

The apparent synchrony of accelerating floodplain accumulation and the floodplain structure is consistent with a model that could include but does not require increased anthropogenic erosion or more, recent flooding episodes.