**3. Methods**

Geodetic and bathymetric measurements of Lake Mladotice were conducted in 1972, 1990 and 2003 (Jansky 1976, 1977; Jansky & Urbanova 1994; Česak & Šobr 2005). By comparing the results of these measurements, it is possible to analyse the dynamics of the lake's sedimentation in the past and to attempt to predict the further development of the lake basin: in other words, to determine a period after which the lake will be entirely filled with sediment.

To reconstruct land use shifts, the Military Topographical Institute in Debrovska (western Bohemia) supplied air photos taken in 1938, 1952, 1975, 1987 and 1998. Up to now only 2-D interpretation has been possible, because of the lack of overlap. The air photographs document a considerable land use change with the introduction of collectivisation (see below). We anticipate that stereoscopic 3-D interpretation of the air photos will show the removal of field terraces that accompanied field enlargement and also contributed to increased erosion (Janský 1976, 1977; Jansky & Urbanova 1994).

Rainfall data were analysed from six stations recording data since 1881 to assess the influence of natural factors on erosion, transport and sedimentation rates in Lake Mladotice. The rain gauges at Kralovice, Plasy, Mladotice, Manetin, Liblin and Valecin are not located in the drainage basin of Lake Mladotice, but they are the closest stations in the surrounding area. These records do not seem to be homogeneous for all events at all stations and need to be examined carefully to identify the rainfall events relevant to flooding. Rainfall runoff analysis should therefore focus on major events which covered the whole area of the rainfall stations, including the drainage basin of Lake Mladotice.

Runoff data serve to indicate the dimensions of past flood events. The nearest runoff gauge is located on the Strela River (Plasy Station, 775 km², Fig. 2 small map), which also drains the Mladoticky creek. Daily records at this gauge date back to 1941. It is assumed that large floods recorded at the Strela River were also experienced at the Mladoticky creek and that sediments were deposited in the lake during these events.

To classify current sedimentation conditions and sediment properties, we measured the oxygen content, conductivity, temperature, visible depth and stream flow of the lake water in the summers of 2003 and 2004 in a depth grid across the lake.

Sediment echo sounding was used to measure sediment distribution along profiles in the lake basin in order to find suitable sites for core drilling. Unfortunately the sediments were extremely poor in reflection owing to the great number of cavities in the lake sediments, and thus results were unsatisfactory. The analysis of the sediment cores confirms the great number of cavities.

In spite of this setback, we obtained information about the distribution of the sediments by extracting 13 short cores, each about 1 m in length. Five long cores were drilled down to the bottom of the sediments. The reference core ML 18/03 was extracted from the deepest part of the lake, and core ML 14/03 was investigated for diatom analyses (Fig. 3).

Reference core ML 18/03, with a total length of 4 m, underwent analyses of water content, density, grain size distribution, total sulphur, total carbon, total phosphorus, clay mineral composition, the isotope content of 137Cs, 241Am and 210Pb as well as thin sections from the entire length of the core. Sediment samples were taken and analysed in 10 cm sections. The upper part of the sediment core ML 14/03 was investigated for diatoms (0–160 cm core depth). As agrochemicals can indicate system changes, we analysed their input into the lake. Fractionated organic analyses were conducted on four samples using GC-MS technology.

Fig. 3. Map of Lake Mladotice showing locations of short and long cores. Core ML 18/03 is located near the outflow with the maximum water depth.
