The peculiarity of small rivers is the dependence of erosion-accumulative processes in their beds on the intensity of soil erosion in the catchment area: the smaller the river, the greater the contact with the catchment area of its channel, where mineral particles, washed off from its area, directly enter it. Soil erosion leads to the entry of an excessive amount of them into the channel of a small river, as a result of which sediment accumulation begins in it, siltation of the channel occurs. It leads to changes in the water regime of small rivers - a decrease in inter-soil runoff (up to the drying up of rivers), a sharp reduction in underground power supply. In the humid zone, due to the high water content, the siltation of small rivers is poorly expressed and has a local character. Siltation of small rivers is especially characteristic for the forest-steppe and steppe zones, as well as for the south of the forest zone. In the steppe zone of the European part of Russia, as a result of siltation, rivers of the first order completely disappeared, up to the fourth (up to 40-50 km long), leading to a total reduction of the river network by up to 30 %. In the forest zone, due to the disappearance of sources, the number of rivers of the first and second orders (up to 20 km long) decreased by 2.2 times. During the agricultural period (about 300 years), a layer of sediment with a thickness of 0.5 to 1 m accumulated in the beds and floodplains of small rivers in the southern half of the European part. The erosion of small riverbeds reaches on average only 20 % of washed off products, which is about 60 % of the total river sediment runoff. The rate of siltation of small rivers in the Don basin is 6-12 mm / year with a length of up to 25 km and about 1 mm/year with a length of 100 km and above. Siltation of small rivers is not typical for regions where the soil is washed away by meltwater, since the maximum flow of sediment into the rivers coincides with high water, when the channel-forming activity of water flows is most active.

The rapid development of society, increase in the population, land urbanization, growth of human economic activity contribute to the involvement in economic turnover of an increasing number of natural resources, the main part of which is water. Along with the depletion of water bodies, their pollution is also intensively occurring. The analysis of ecological state of basins of small rivers in the South of Russia shows a constant increase in the emission of pollutants into their channels. On the territory of Rostov region there are 5.572 rivers with a total length of 36.064 km. 90 % of rivers of Rostov region are less than 10 km long. On average, a local runoff in the amount of 3.36 billion m3 is formed on the territory of Rostov region per year. Increased anthropogenic activity leads to the disruption of functioning and stability of natural systems, which has a significant impact on small rivers, which are the most common elements of the hydrographic network. About 30 % of urban and about 90 % of rural population live in catchments of small rivers, while the greatest density is observed in the immediate vicinity of water banks. Small rivers significantly affect the hydrological, biological and biochemical regime of landscapes, maintaining the balance and redistribution of moisture and are important for water supply of the population.

Currently, it is necessary to assess competently and timely the technical condition of long-operated bridge crossings on the main canals of irrigation systems with increasing loads. The article considers structures with various defects and damages by the example of the Abinsky irrigation system of the Lower Kuban, examines the history of the issue of assessing the remaining resource of bridge structures. It includes the values of coefficients of significance of bridge crossing structures on the main canals of irrigation systems obtained by the authors based on the results of calculation. When designing irrigation systems, the estimated service life was no more than thirty years. Currently, on average, more than fifty years have already passed since the commissioning of the main irrigation systems, for example, in Krasnodar Territory. At the moment, the determination of the true value of the remaining resource is massively in demand not only in the construction field and hydraulic engineering, but also in many other branches of industrial production. For more effective control over the technical condition of bridge crossings on the main canals of irrigation systems, scientific and practical interest is primarily not only an understanding of the actual condition of the structure at the time of the survey, but also its alternative change, i.e. the forecast of its remaining resource. Assessment of the technical condition of the bridge crossing structures on the main canal is determining the possibility of their further long-term operation, the need for strengthening, reconstruction of bridge crossings on the main canals of irrigation systems.

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