Fig. 2: North American alluvium accumulation rates versus age.
From: The human impact on North American erosion, sediment transfer, and storage in a geologic context
Raw data are shown as gray circles (note logarithmic scales). Colored datapoints are the median rates and ages at each study site, with color-coding based on the timespan of measurement. Note the tendency for rates measured over short timespans to be faster (particularly apparent in rates aged <100 y), illustrating how rates are partially biased by timespan dependence (see text and Fig. 3a for details). Nevertheless, there is a stepwise increase in rates between the approximate pre- and post-settlement time periods ~200 y ago (~1820 CE). This increase coincides with a marked expansion in total agricultural land area in North America (blue line, data from ref. 23) and the onset of rapid population growth (red line, data from ref. 45. USA population prior to 1790 includes only white Europeans45). Agriculture and population increases are bracketed by an interval of intensive milldam construction in both Canada and USA (dates from refs. 5,24,25). The stepwise increase in accumulation rates ~200 y ago is not attributable solely to timespan dependence, and reflects an anthropogenic impact on alluvium sedimentation.
