Background
The authors write that "long-term decrease in seasonal peaks of water levels allows the settling of relatively low geomorphic locations, such as river and lake floodplains, while a rise in flood levels causes settlements to be shifted to higher elevations," based on the logical assumption that "ancient settlements could not persist under the impact of regular inundations."

What was done
In a study of the Upper Volga and Zapadnaya Dvina Rivers of Russia, Panin and Nefedov documented "the geomorphological and altitudinal positions of [human] occupational layers corresponding to 1224 colonization epochs at 870 archaeological sites in river valleys and lake depressions in southwestern Tver province," identifying "a series of alternating low-water (low levels of seasonal peaks, many-year periods without inundation of flood plains) and high-water (high spring floods, regular inundation of floodplains) intervals of various hierarchial rank."

What was learned
The two Russian researchers report finding that "low-water epochs coincide with epochs of relative warming, while high-water epochs [coincide] with cooling epochs," because "during the climate warming epochs, a decrease in duration and severity of winters should have resulted in a drop in snow cover water equivalent by the snowmelt period, a decrease in water discharge and flood stage, and a decrease in seasonal peaks in lake levels," noting that "a model of past warming epochs can be the warming in the late 20th century, still continuing now." They also report finding that "in the Middle Ages (1.8-0.3 Ky ago), the conditions were favorable for long-time inhabiting [of] river and lake floodplains, which are subject to inundation nowadays [italics added]." In addition, their results indicate that of this time interval, the period AD 1000-1300 hosted the greatest number of floodplain occupations.

What it means
Interestingly, Panin and Nefedov state that this last period and other "epochs of floodplain occupation by humans in the past can be regarded as hydrological analogues of the situation of the late 20th-early current century," which they say "is forming under the effect of directed climate change." And this relationship clearly implies that the current level of warmth in the portion of Russia that hosts the Upper Volga and Zapadnaya Dvina Rivers is not yet as great as it was during the AD 1000-1300 portion of the Medieval Warm Period.