Atmospheric precipitation falling upon the land surface is partly evaporated, partly retained within the soil, and partly flows toward lower areas of the land, forming both surface runoff and groundwater runoff. As water moves across the surface, it expends considerable amounts of energy in overcoming internal friction and external resistance to movement, the latter arising chiefly from the irregularities of channel and boundary surfaces. In the process, water performs enormous work by washing away and eroding rocks and soils, transporting various substances of mineral and organic origin, both as sediments and dissolved matter, to lower parts of the Earth's surface and, in part, carrying them into the oceans. The work performed by flowing water determines the intensity and character of water erosion, that is, the degree to which runoff acts upon the weathering mantle of the land surface. Three principal types of water-erosion processes must be distinguished¹: The washing away, erosion, and dissolution of soils and rocks, resulting in the formation of the primary material that becomes the source of solid runoff and chemically dissolved substances; The migration (transport) of erosion products by surface water and groundwater from their place of origin to lower parts of the land surface; The deposition (accumulation) of transported erosion products along the path of flow, wherever, for one reason or another, the carrying capacity of the runoff becomes insufficient to transport all of the eroded material in motion. It should be emphasized that these three types of water-erosion processes occur continuously and simultaneously throughout the entire course of surface and subsurface runoff, although with varying intensity. ¹ In this work, water erosion is understood as the totality of all processes by which flowing water modifies the land surface, including both erosion and the deposition of eroded material. It is self-evident that washing and erosion are much more intense during flood periods, when large quantities of water reach the surface in a more concentrated manner. Within any drainage basin, flooding may occur almost simultaneously across the entire basin area, as during spring snowmelt or prolonged rainfall, or may occur at different times in different locations, as during localized storm precipitation. Deposition of transported material may likewise occur throughout the entire runoff pathway, beginning in depressions within the basin and low-lying portions of slopes while slope runoff is still in progress, and continuing in floodplains, widened river channels, and lakes belonging to the corresponding river system. Nevertheless, because of the specific characteristics of each type of process, it is customary to consider them in the sequence outlined above. The first type of water erosion, namely washing, erosion, and dissolution, depends to a considerable degree upon the resistance of soils and rocks to the erosive action of flowing water, as well as upon the climatic characteristics of the region. Through weathering, the upper layers of rocks disintegrate into increasingly fine particles. However, for weathering products to become material forming solid runoff, additional factors are required, chiefly the presence of surface runoff of sufficient magnitude and water velocities capable of transporting the material. The second type of erosional process, namely the migration of products generated by washing, erosion, and dissolution, may be regarded as equivalent to the transport of solid runoff and dissolved substances. Solid runoff, including suspended load and bed load, is formed both from products of physical weathering that are washed from hillslopes within the drainage basin and from erosion products derived from the channels constituting the river system. Chemically dissolved substances enter channel runoff both through dissolved material carried by groundwater and through the dissolution of sediments during their movement along slopes and within river channels. Both solid runoff and dissolved-substance runoff contain components of mineral and organic origin. The relative proportions of mineral and organic material, as well as the magnitude of solid runoff compared with dissolved loads, depend upon the natural-geographical characteristics of the drainage basin, principally: the nature of climate and vegetation; the resistance of soils and unconsolidated deposits to mechanical and chemical action by water; the quantity and chemical composition of groundwater recharge; the transporting capacity of surface runoff; and other related factors. The third type of water-erosion process, the accumulation of sediments, as noted above, becomes evident already during the initial stages of erosion-product transport, namely on the slopes of the drainage basin itself. Sediment accumulation may also be observed during transport along river channels. This is related both to the general characteristics of the longitudinal profile and to particular sections where stream energy decreases and transport capacity becomes insufficient, at least during part of the year. Such locations include widened valley reaches, where shoals commonly develop; lakes belonging to the river system; floodplain terraces; oxbow lakes; and the lower reaches of rivers, especially near their mouths. Products of crustal weathering are transported from their places of origin to other locations chiefly through the action of wind and flowing water. Other forms of transport affecting weathering products and soil particles, such as rockfalls, landslides, debris flows, and similar processes, may at times attain considerable magnitude, but in most cases remain local and spatially limited in extent. For the purpose of analyzing erosion of soils and unconsolidated deposits, a schematic diagram of erosion processes applicable to an individual permanent drainage basin was prepared (Fig. 1). Part of the erosion products remains within the drainage basin. It is therefore necessary to distinguish this component, referred to as “local” migration, from the portion that is transported beyond the boundaries of the basin, referred to as “transit” migration. Accordingly, assuming that the crustal block within the basin boundaries remains fixed, that is, experiences no vertical displacement, the general balance equation for erosion processes within a river drainage basin takes the following form: ΔM=−[Rt+Gt+St+Vt−Vt′+Eu]\Delta M = -[R_t + G_t + S_t + V_t - V_t' + E_u] where: ΔM = change in the volume of material above the erosion base level within the drainage basin caused by erosion processes; Rₜ = products of washing and erosion from basin surfaces and channels removed beyond the drainage basin in the form of suspended load; Gₜ = water-erosion products removed beyond the basin in the form of bed load; Sₜ = water-erosion products removed beyond the basin in dissolved form. where: Vₜ = the portion of aeolian erosion products removed beyond the boundaries of the drainage basin by wind; V′ₜ = aeolian erosion products transported into the basin from neighboring drainage basins; Eₙ = products of other forms of erosion (apart from water and wind erosion), such as landslides, rockfalls, debris flows, and similar processes, which are carried beyond the boundaries of the drainage basin. Figure 1. Schematic representation of soil and unconsolidated-material erosion processes. M = local migration; T = transit migration. The diagram distinguishes: aeolian erosion (V); water erosion (W); other forms of erosion (N) such as landslides, rockfalls, and debris flows; local migration within the basin; transit migration beyond the basin; material introduced from neighboring basins; changes occurring on the basin surface; modifications of temporary and permanent stream channels; chemical dissolution of soils and rocks by surface and groundwater; debris-flow processes; water-erosion products, including bed load (g), suspended load (R), and dissolved substances (S), each of mineral and organic origin. All terms in the above equation are expressed in volumetric units. Of the total quantity of erosion products transported by surface runoff, only a relatively small fraction travels long distances. Most of the material is trapped along the way by irregular topography and vegetation cover, gradually filling depressions and forming colluvial and alluvial deposits at the foot of hills and mountains. The magnitude of the various forms of soil and unconsolidated-material erosion described above depends upon the natural-geographical conditions of a given region. Consequently, their relative proportions may vary within very wide limits. Nevertheless, it may be stated with confidence that in large or moderately large drainage basins the magnitude of water erosion substantially exceeds that of other erosion types, especially with regard to the “transit” component of transported material. During the last two decades, processes of soil and sediment erosion have attracted increasing attention in the Soviet Union among researchers representing various scientific disciplines because of their strong influence on numerous fields of practical human activity. For example, erosion damages and depletes cultivated lands that have been created and maintained through the labor of many generations. Combating this phenomenon is therefore an urgent task confronting soil scientists, agronomists, and other agricultural specialists. Soil and sediment erosion is the principal source of solid runoff. Its magnitude and dynamics strongly influence many processes of major practical importance, such as the silting of lakes, reservoirs, and canals, the formation and transformation of deltas and shoals, and the clogging of water intakes. Particular attention must be paid to destructive debris flows in mountainous regions. Hydrological studies of solid-runoff processes contribute to the development of appropriate river-protection measures by hydraulic engineers, hydroelectric specialists, and other professionals. As a consequence of erosion and solid runoff, some areas of the Earth's surface undergo lowering, while others experience the accumulation of unconsolidated material. Landforms are reorganized, resulting in changes to the existing distribution of loads acting on different parts of the Earth's crust. Data on solid runoff and its depositional products assist in clarifying questions concerning the age and future evolution of mountain ranges, lakes, alluvial plains, and river basins (A. F. Volin, 1946). Erosion products enter rivers, lakes, and seas in the form of sediments and dissolved substances. A portion of these materials serves as nutrients for aquatic plants and animals inhabiting these water bodies. However, where solid-runoff loads are very large, conditions for biological development may deteriorate substantially. For this reason, the problems associated with solid runoff have attracted the interest of biologists and hydrobiologists of various specialties (V. I. Zhadin, 1946). The study of water erosion is of particularly great significance at the present time because the extensive hydraulic-engineering projects envisaged under the Grand Stalin Plan for the Transformation of Nature are being implemented. The measures proposed under this program are expected to reduce both water and wind erosion, thereby contributing significantly to improved soil quality and enhancing the resistance of the numerous ponds and reservoirs constructed for irrigation purposes against sedimentation. The present work is devoted primarily to the first and second types of water-erosion processes, namely: the formation of movable solid particles (sediments) through washing and erosion, and the transport (migration) of these sediments from their place of origin to their place of deposition.