Historic sediment accumulation rate development was investigated in Karlskärsviken, a bay of Lake Mälaren, by dating of sediments in cores taken from the bay. The Lake Mälaren transition period from brackish water to freshwater was determined by diatom analysis and for modern times by fly-ash (SCP) and caesium isotope Cs-137 analyses. Results show that in the long period from medieval times to about 1950, the sediment accumulation rate increased to about double in the outer section of the bay, but only by about 33% in the near shore. After that, new farmland was drained by a ditch dug in 1951 with its outlet in Karlskärsviken, which enlarged the bay catchment area by 100%, and a marina and a public beach were constructed in 1953. These catchment system changes increased considerably the inflow of particles to Karlskärsviken and the sediment accumulation in the bay, directly and mostly in the near shore, but also with some and longer term effects further out in the bay. In later times, as the ditch became overgrown by reeds and bushes, the sedimentation rate decreased again in the near shore, while it continued to increase further out in the bay. These results indicate that even relatively small and common catchment enterprises may considerably affect sediment transport to recipient bay waters.

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