33 M. G¹siorowski and H. Hercman RECENT CHANGES OF SEDIMENTATION RATE IN THREE VISTULA OXBOW LAKES DETERMINED BY 210 Pb DATING MICHA£ G¥SIOROWSKI and HELENA HERCMAN Institute of Geological Sciences, Polish Academy of Sciences, 00-818 Warsaw, ul. Twarda 51/55 (e-mails: mgasior@twarda.pan.pl, hhercman@twarda.pan.pl) Abstract: Abstract: Abstract: Abstract: Abstract: Sediments of three oxbow lakes located in Vistula valley near Warsaw (Poland) were analysed for activity of 210 Pb. The sediment age was calculated applying the CRS model. The sedimentation rate was determined for each lake based on the sediment age and thickness under the assumption of constant deposition between dated layers. Sedimentation rate varies in time and between lakes. The major changes of sedimentation rate were corre- lated with flood events and construction of flood dams that isolate the lakes from the river. The recent differences between the lakes seem to be related to different trophy state and pro- ductivity. Key words ey words ey words ey words ey words: OXBOW LAKES, VISTULA, 210 Pb DATING, SEDIMENTATION RATE GEOCHRONOMETRIA Vol. 24, pp 33-39, 2005 – Journal on Methods and Applications of Absolute Chronology 1. INTRODUCTION Oxbow lakes and floodplain wetlands are very impor- tant elements of a river system. The different generations of oxbows are usually located differently in relation to the river. The river supplies these lakes with detrital matter and nutrients during the short periods of floods. Between these episodes, the lakes develop in distinct manner. This results sometimes in a different character of even closely located lakes. The flood events could be recorded in ox- bow sediments. According to the energy of the floodwa- ter, the lake sediments could be eroded or buried by flu- vial sediments. Therefore, the flood events are recorded not only by the changes of sediment type, but also by the changes of the sedimentation rate. 210 Pb dating is one of the most powerful methods for the establishment of chronologies in recent lake sediments (Goldberg, 1963; Krishnaswami et al., 1971). It was pos- sible to apply it to dating the youngest sediment (T 1/2 = 22.26 yr) deposited during the last two centuries and to tracking processes of mixing, focusing and re-deposition (San Miguel et al., 2003; Sonke et al., 2003). Consequently, this method seems to be one of the best for tracking flood events recorded in oxbow lakes’ sediments. 2. STUDIED SITES The Vistula valley near Warsaw is diversified by a lot of small lakes. Their genesis is related to the cutting off of the river channels. All these water bodies have some similarities; they are strongly elongated, shallow (maxi- mum depth 2-4.5 m), and small (up to 10 ha). The lakes’ location, close to the city, strongly affects its present tro- phy state, water level, flora, and fauna. Three Vistula oxbow lakes were selected for the present study (Fig. 1). Two of them, Kazuñskie Górne (KAG) and Kazuñskie Dolne Lake (KAD), are closely located lakes situated northwest of Warsaw. Third, Lisowskie Lake (LIS) is located in southern part of the Warsaw metropolis. The lakes are located at a flood ter- race at an altitude of 71.1 (Kazuñskie Lakes) and 83.8 m a.s.l. (Lisowskie Lake), but have no connection to the river today because of artificial flood dams.