As part of an archaeological excavation in Valdštejnská street in the Lesser Town of Prague, flood sediments in an old channel of the river Vltava were studied by means of pollen analysis. Analyses were performed on a core taken before the archaeological excavation and samples from the layers uncovered by the excavation. The core includes deposits from the era that followed the construction of weirs in the second half of the 13th century up to approximately the 15th century. Some of the sediments are older and from Early Medieval times (the oldest from the end of the 10th century). For the pollen analysis, three types of sediment were studied: flood loams, cultural layers and material deposited on causeways. Thanks to the diversity in the sediments it was possible to study local and regional components of the pollen spectra in more detail. The vegetation growing in the old river channel consisted of ruderal and weed taxa with sedge stands surviving in less accessible places. This locality most probably did not serve as a dumping ground until at least the 14th century, and even then this is not directly indicated by the pollen analysis. The difficulty of interpreting the mixed-origin pollen spectra usually present in urban archaeobotanical deposits is a common problem. Using multivariate statistics, three groups of pollen taxa characteristic for each particular sediment type were separated, and the individual pollen sources (and corresponding taphonomical processes) partly separated. Therefore, it was possible to distinguish autochthonous and allochthonous sources of pollen and draw conclusions about the local vegetation at this site.