Rivers in the Eastern Mediterranean, Israel in particular, are small and located mostly in the coastal plane. In the first step of ecosystem assessment, 671 species of algae and cyanobacteria belonging to nine taxonomic divisions were revealed by our research during 2002-2009 in the Yarqon, Alexander, Hadera, Qishon, and Oren rivers in the coastal plane, the Lower and Upper Jordan Rivers in the rift valley, as well as the Zin stream in the Negev Desert. This book presents and discusses research which suggests that in semiarid environments the influence of anthropogenic factors is strongly modulated by climatic impacts causing a marked decrease of species richness in aquatic ecosystems from north to south.