**4.1 Sediment filling in the lake basin and fluctuations of lake water level**

Comparative analysis of bathymetric measurements from 1972 and 2003 yielded the following results (Jansky 2003). The maximum depth of the lake decreased from 7.7 m to 6.7 m (Fig. 4). The 7 m depth level disappeared entirely, and the area of all other depth levels decreased - the 6 m depth level to 61% of its initial area from 1972, the 5 m level to 43%, the 4 m level to 60%. The decline in the area of shallow water levels was somewhat less dramatic – the 3 m level decreased to 72% of its 1972 area, while the 2 m and 1 m levels decreased to 69% and 76%, respectively.

A decrease in the water level's surface area was measured, i.e. from an initial 5.85 ha (1972) to 4.73 ha (2003). This means a decrease of 1.12 ha in the lake's surface area, i.e. 19% of its initial area in 1972. The maximum water level fluctuation between 1972 and 2009 was recorded at around 55 cm. Moreover, the automatic limnigraph has measured a fluctuation of 26 cm in the last 12 months. After the bathymetric curves were elicited, the water volume of the lake basin was calculated. From an initial volume of 141,380 m³ in 1972, it decreased by 37,471 m³ to 103,910 m³; the water volume of the lake decreased by 26.5%.

Lake Mladotice in the Western Czech Republic – Sediments as a Geoarchive

between 1978 and 2002.

1975.

for Flood Events and Pre- to Postcommunist Change in Land Use since 1872 313

the years (Fig. 6). The number of floods above a threshold of 5.5 m³/s increased slightly from 4.1 flood events per year (1941-1956) to 5.0 (1957-1977) and 5.1 flood events per year

Fig. 5. Air images of a field about 1 km² in size, northeast of the town of Zihle in the basin of Lake Mladotice (see Fig. 2). Collective farming had the greatest impact between 1952 and

Fig. 6. Flood events at Plasy gauging station between 1941 and 2002, showing floods that

exceed the threshold of 5.5 m³/s (pot = peaks over threshold).

Fig. 4. The bathymetric maps of the Lake Mladotice from measurements in 1972 and 2003

## **4.2 Changes in land use and flood discharge**

Landscape changes in the drainage basin of Lake Mladotice were reconstructed from air images. To visualise the land use changes, Fig. 5 displays as an example a field about 1 km² in size that is located northeast of Žihle (for orientation see Fig. 2). No changes are visible in the field patterns between 1938 and 1952. Collective farming had the greatest impact between 1952 and 1975, when fields were made much larger. A further increase in the size of some fields is visible in 1987. The photos taken in 1998 show that the size of the fields was reduced again after the political change in 1989. Bigger fields facilitate soil erosion due to longer slopes and increased surface runoff (see conclusions). Some quantitative data about land use changes were published in Schulte et al. (2006).

To clarify whether the system changes are due to natural or anthropogenic causes, we analysed the time series of discharge values at the Střela gauge at Plasy (775 km²) from 1941 until 2002 (2003 is the year of sediment coring). Plausibility and homogeneity checks of the discharge data revealed discontinuities and varying trends in the years 1956 and 1978, so further studies were made in three separate periods (1941-1956, 1957-1977 and 1978-2002). In these three periods, the annual flood peaks show a falling tendency, i.e. a lower peak over

Fig. 4. The bathymetric maps of the Lake Mladotice from measurements in 1972 and 2003

Landscape changes in the drainage basin of Lake Mladotice were reconstructed from air images. To visualise the land use changes, Fig. 5 displays as an example a field about 1 km² in size that is located northeast of Žihle (for orientation see Fig. 2). No changes are visible in the field patterns between 1938 and 1952. Collective farming had the greatest impact between 1952 and 1975, when fields were made much larger. A further increase in the size of some fields is visible in 1987. The photos taken in 1998 show that the size of the fields was reduced again after the political change in 1989. Bigger fields facilitate soil erosion due to longer slopes and increased surface runoff (see conclusions). Some quantitative data about

To clarify whether the system changes are due to natural or anthropogenic causes, we analysed the time series of discharge values at the Střela gauge at Plasy (775 km²) from 1941 until 2002 (2003 is the year of sediment coring). Plausibility and homogeneity checks of the discharge data revealed discontinuities and varying trends in the years 1956 and 1978, so further studies were made in three separate periods (1941-1956, 1957-1977 and 1978-2002). In these three periods, the annual flood peaks show a falling tendency, i.e. a lower peak over

**4.2 Changes in land use and flood discharge** 

land use changes were published in Schulte et al. (2006).

the years (Fig. 6). The number of floods above a threshold of 5.5 m³/s increased slightly from 4.1 flood events per year (1941-1956) to 5.0 (1957-1977) and 5.1 flood events per year between 1978 and 2002.

Fig. 5. Air images of a field about 1 km² in size, northeast of the town of Zihle in the basin of Lake Mladotice (see Fig. 2). Collective farming had the greatest impact between 1952 and 1975.

Fig. 6. Flood events at Plasy gauging station between 1941 and 2002, showing floods that exceed the threshold of 5.5 m³/s (pot = peaks over threshold).

Lake Mladotice in the Western Czech Republic – Sediments as a Geoarchive

these elements at a core depth of 100 cm.

**4.4 Analyses of isotopes and diatoms** 

the extreme magnitude of the flood in 1978.

dotted lines.

1986.

for Flood Events and Pre- to Postcommunist Change in Land Use since 1872 315

system change occurred at different depths owing to the different thickness of the sediments. The sediment chemism of core ML 14/03 shows a distinct increase of TC and TS above a core depth of 200 cm (TS increases sixfold); core ML 16/03 shows the buildup of

Fig. 8. Heavy metal contents in reference core ML 18/03 (median of grain size, Mn, Fe, Pb, Cu, Ni, Cr, Zn). Changes in the heavy metal concentrations or the level are marked with

The absolute chronology of the sediments is also based on available isotope measurements of 137Cs, 241Am and 210Pb (Fig. 9). The peak radiation of 137Cs and 241Am at a core depth of 100 cm is attributed to the 1963 maximum of bomb fallout which started in 1954. Americium clearly demonstrates bomb fallout because there was no emission of americium during the Chernobyl disaster. The peak at 40 cm core depth is assigned to the Chernobyl fallout in

Analyses of microfloral and faunal remains confirmed the system change between the upper and lower parts of the reference core (transition at 190 cm). However, a very high frequency of diatoms was found in the upper part of the core. Samples taken from core ML 14/03 from the sediment surface down to 166 cm core depth (location see Fig. 3), indicate that about 80-90 % of the individuals are planktonic and the remaining 10-20% are benthic diatoms. This uniform palaeolimnological stratification is interrupted by one distinct event at a depth of 66-76 cm, where the proportion of planktonic individuals drops to 15 % and the benthic forms increase to a peak of 85 %. This event indicates a high sediment inflow during a major flood. The analysis of the runoff data indicates that this big event relates to

Against the background of the contrary trends of magnitude and frequency of the time series of annual flood events, it does not seem possible to infer decreasing or increasing sedimentation in the lake. During the entire 1941-2002 period, only the 1978 flood is notable for having the highest peak discharge in the entire measuring period; accordingly, it has left a distinct event layer in the lake sediments (see below).

#### **4.3 Stratigraphy and geochemistry of the lake sediments**

The lake sediments of reference core ML 18/03 (location see Fig. 3) are largely muddy silts. The particle-size distribution indicates two noteworthy features: 1. Sand is found only in the lower sediment sequences. This is also the case in the other sediment cores and suggests that the sand was brought in by the Mladoticky creek. During the early decades there may have been some additional sediment input from the mass failure area, which was unvegetated during the first few years. 2. The particle-size median shows a distinct change in sedimentation at about 190 cm core depth. Below this depth, the sediment is coarser and the range fluctuates fairly widely; above it, the median remains constant at about 4 µm.

The sediment chemism of reference core ML 18/03 is shown in Fig. 7. Some of the contents of carbon (TC), phosphorus (TP) and sulphur (TS) double above a core depth of 190 cm. This system change is demonstrated even more clearly by the heavy metal levels. Pb, Cu, Ni and Zn rise sharply above 190 cm (Fig. 8). Other elements such as calcium (Ca) show an increase only in near-surface sediments above 60 cm, which correlates with the occurrence of calcite. Also TC, TP and TS show a marked increase near the surface. Owing to the phosphate content of the open water, the lake can nowadays be classified as eutrophic.

Fig. 7. Geochemistry of reference core ML 18/03 (median of grain size, C, S, N, C:N, Ca, Mg, K, Na, P). Changes in the element concentrations or the level are marked with dotted lines.

Sediment cores ML 14/03 and ML 16/03 (location see Fig. 3) show clear evidence of the system change. According to macroscopic and stratigraphic analyses of these cores, the

Against the background of the contrary trends of magnitude and frequency of the time series of annual flood events, it does not seem possible to infer decreasing or increasing sedimentation in the lake. During the entire 1941-2002 period, only the 1978 flood is notable for having the highest peak discharge in the entire measuring period; accordingly, it has left

The lake sediments of reference core ML 18/03 (location see Fig. 3) are largely muddy silts. The particle-size distribution indicates two noteworthy features: 1. Sand is found only in the lower sediment sequences. This is also the case in the other sediment cores and suggests that the sand was brought in by the Mladoticky creek. During the early decades there may have been some additional sediment input from the mass failure area, which was unvegetated during the first few years. 2. The particle-size median shows a distinct change in sedimentation at about 190 cm core depth. Below this depth, the sediment is coarser and the

The sediment chemism of reference core ML 18/03 is shown in Fig. 7. Some of the contents of carbon (TC), phosphorus (TP) and sulphur (TS) double above a core depth of 190 cm. This system change is demonstrated even more clearly by the heavy metal levels. Pb, Cu, Ni and Zn rise sharply above 190 cm (Fig. 8). Other elements such as calcium (Ca) show an increase only in near-surface sediments above 60 cm, which correlates with the occurrence of calcite. Also TC, TP and TS show a marked increase near the surface. Owing to the phosphate

Fig. 7. Geochemistry of reference core ML 18/03 (median of grain size, C, S, N, C:N, Ca, Mg, K, Na, P). Changes in the element concentrations or the level are marked with dotted lines. Sediment cores ML 14/03 and ML 16/03 (location see Fig. 3) show clear evidence of the system change. According to macroscopic and stratigraphic analyses of these cores, the

range fluctuates fairly widely; above it, the median remains constant at about 4 µm.

content of the open water, the lake can nowadays be classified as eutrophic.

a distinct event layer in the lake sediments (see below).

**4.3 Stratigraphy and geochemistry of the lake sediments** 

system change occurred at different depths owing to the different thickness of the sediments. The sediment chemism of core ML 14/03 shows a distinct increase of TC and TS above a core depth of 200 cm (TS increases sixfold); core ML 16/03 shows the buildup of these elements at a core depth of 100 cm.

Fig. 8. Heavy metal contents in reference core ML 18/03 (median of grain size, Mn, Fe, Pb, Cu, Ni, Cr, Zn). Changes in the heavy metal concentrations or the level are marked with dotted lines.
