Lake Mladotice in the western Czech Republic - sediments as a geoarchive for flood events and pre- to postcommunist change in land use since 1872
Abstrakt
Lake Mladotice is of a unique genetic type in Czechia. In May 1872, a landslide occurred as a result of an extreme rainfall event; the accumulated rock masses blocked the Mladoticky creek (drainage basin 46 km2, 50% arable land) and dammed Lake Mladotice.
The 1952 and 1975 air images of the drainage area document that collective farming, which was practised until 1987, had a great impact on the lake basin evolution when balks and field terraces were removed and fields were made much larger. Because of this change in land use, an increase in soil erosion was expected as well as a related rise in the sedimentation rate in the lake.
First bathymetric measurements were carried out in 1972 and were repeated in 1999 and in 2003. Irrespective of changing land use, changes in flood frequency and/or magnitude are capable of increasing soil erosion and sedimentation rates in the lake.
Over the whole measuring period, the nearest gauging station downstream of the lake indicates a rising trend in the frequency but a falling trend in the magnitude of the flood events } 5.5 m3/s (peak over threshold series). Only the 1978 flood discharge was deposited in a distinct layer in the lake sediments.
Our analysis of the sedimentary record aims to identify the factors controlling sediment input, i.e. soil erosion, which is dependent on rainfall-runoff processes, and land use in the drainage basin. The sediment stratigraphy, physics, chemistry, micropalaeontology, isotope content and thin sections yield a detailed temporal resolution of the sedimentation chronology.
Surprisingly, the sedimentation rates indicate only a small increase from 2.2 cm/a (1954-1963) to 2.7 cm/a (1963-1978) to 2.5 (1978-1986) and 2.4 cm/a (1986-2003). This suggests that colluvial, alluvial and lake inflow deposits may have acted as (temporary) sinks.