The rapid and intensive development of the productive forces in the eastern regions of our country has created many complex tasks for the scientific research, surveying and design, and industrial organizations operating there. The construction of large energy, industrial, transportation, and other integrated complexes requires their rational placement with full consideration of all natural conditions, including hydrological factors. However, the hydrological conditions of southern Eastern Siberia remain insufficiently studied, and detailed descriptions of river runoff characteristics, especially spring freshets and floods, are lacking. This book is intended to help fill this gap by introducing readers to a number of issues related to river runoff formation and hydrological conditions in the region. It presents characteristics of the rivers of the Eastern Sayan Mountains and the Khamar-Daban Range, with particular attention given to the conditions governing the formation and distribution of summer floods and mudflows in the Angara River basin and along the southern shores of Lake Baikal. The mountainous regions of the Eastern Sayan and Khamar-Daban are located in the southern parts of Krasnoyarsk Krai and Irkutsk Oblast and in the western part of the Buryat Autonomous Soviet Socialist Republic. They form the headwaters of numerous mountain rivers, including the Yenisei, Angara, and Selenga Rivers and their tributaries, as well as the drainage system of Lake Baikal. The total area of the region exceeds 200,000 square kilometers and constitutes part of the basin of the Yenisei, the largest river system in the country. Research on this region is of considerable practical and scientific importance. The study area is characterized by abundant mineral and other natural resources and already plays an important role in the development of the national economy. The water resources of its numerous rivers are also of great significance, although they are currently utilized only to a limited extent.