ABSTRACT
While the natural history of the gully, tributary, and upper main valley zones was quite complex with alternate filling and cutting, the process in the lower main valley has been somewhat simpler: continuous accretion since shortly after settlement. To be sure, however, those rates varied greatly, but systematically. Using soil profiles, old surveys, excavations of bridges, roads, railroads, and buildings, along with archaeological methods, valley accretion rates were reconstructed for six locations in the lower main valley of Coon Creek (Chapter 3, Figure 3.15). Rates of vertical accretion increased from virtually zero at the time of settlement to about 6 in. (15 cm) per year in the 1920s and 1930s. My investigations since 1973 suggest that this chronology of deposition held over much of the Hill Country, although maximum rates differed locally. Recalling the settlement chronology from Chapter 2, it should be kept in mind that sedimentation peaks may have started a decade or so earlier to the south and perhaps a bit later to the northwest in Minnesota. While Figure 3.15 extends to only 1975, measurements for the period 1975-1993 showed that these floodplains were accreting at only about 3% of the highest rates, a tribute to soil conservation (Trimble, 1999, 2009a; Figure 3.9). Although the present average accretion rate may be low, it is important to realize that these rates are continuing, with some of the sediment migrating from storage in upstream reaches as seen in Chapter 6. The peril remains. It is a slow but sure process. With these rates as a conceptual model, we shall now see the effect on various elements of human settlement. It is in this zone that more human activity took place, and it was where much of the historical sediment accumulated, so it is appropriate to devote more attention to it. And for the same reason, much more documentation is available. Many settlements of the Hill Country were located in valleys, generally along streams on low to high alluvial terraces. Valleys provided excellent agricultural land, dependable water supplies, mill sites, and transportation routes. That the collective intelligence of villagers perceived their settlement sites to be safe from floods (and, indeed, they were safe for over a half century) gives us some prima facie evidence about the generally benign hydrologic regime existing during the second half of the 19th century.
