Qualitative and quantitative parameters of the dynamics of the steering network of elementary watercourses in the northern forest-steppe in the XXI century (on the example of the catchment basin of the Ketarsha river, Privolzhskaya upland, Russia

.


Introduction
The forest-steppe zone is a territory that has extremely comfortable conditions for farming, primarily a favorable climate and fertile chernozem and gray forest soils.In the XXI century, the forest-steppe continues to develop environmental problems caused by anthropogenic transformation of landscapes, reduction of natural landscape areas, pollution of surface and groundwater, the problem of determining scientifically sound priorities and directions of nature management within river basins is acute.
Watercourses of the first order are the initial element of the river network and have a fundamental influence on the formation of river flow, it is this circumstance that determined the purpose of our research.
An elementary watercourse is a watercourse of the 1st ordera small watercourse that has no tributaries.The order of the watercourse increases by one when merging singleorder watercourses.At the same time, the confluence of each tributary of a smaller order is not taken into account [1].
Elementary watercourses react sensitively to changes in landscape conditions.During the period under review (2001-2023), climatic characteristics, hydrographic network objects underwent some changes (ponds were created or eliminated), soil and vegetation cover was actively affected as a result of plowing and haymaking; forest cover -a significant and strongly determined by economic activity factor of the dynamics of the length of the channel networkalso underwent some changes.The lithogenic base and, to a lesser extent, the relief are static (within the period under consideration).

Materials and methods
The purpose of the research: to establish the dynamics of the length of the channel network of elementary watercourses in the catchment area of the Ketarsha River (northern part of the Privolzhskaya upland, Nizhny Novgorod region).
Object of research: elementary watercourses of the catchment area of the Ketarsha River (Privolzhskaya Upland, Nizhny Novgorod region).
Subject of research: dynamics of the length of the channel network of elementary watercourses.
The Ketarsha River basin, within which the research was conducted, has landscape characteristics, typical of the north of the forest-steppe zone within the Privolzhskaya upland, which makes it possible to consider the studied territory representative and extrapolate the results obtained to the rest of the Privolzhskaya upland within the northern forest-steppe.
Research methods.Hydrological researches in the catchment area of the Ketarsha River are based on the results of the analysis of thematic maps, literature, Earth remote sensing data and field researches, conducted by the authors in 2023.
To establish the parameters of the dynamics of the length of the channel network of elementary watercourses, the authors found the current position of the sources of elementary watercourses during the low water period and fixed it using a GPS navigator.Then these data were imported into the geoinformation project and compared with the position of the sources of elementary watercourses displayed on a 1:50 000 scale topographic map, published in 2001, which made it possible to draw conclusions about the direction of the dynamics of the length of the channel network of elementary watercourses and calculate its quantitative indicators.
Since in order to achieve this goal, it was necessary to conduct a comprehensive analysis of spatial data, we actively used the geoinformation method, widely used in conducting geographical research [2,3,4].Spatial analysis, calculations, generalization of data were carried out using Quantum GIS.
Earlier, researches of the length of the dynamics of the channel network in the foreststeppe zone were conducted in various parts of the forest-steppe zone of the Russian Plain [10][11][14][15][16][17].

Results
The territory of the catchment basin of the Ketarsha River is located in the north of the Privolzhskaya Upland in the southeastern part of the Nizhny Novgorod region in the basin of the Pyana River (fig.1).The catchment area of the Ketarsha River is 187 km 2 .
The geological structure of the sedimentary cover includes the thickness of the oldest pre-quaternary sediments, overlain by modern sediments of Quaternary age.The prequaternary formations of the axial part of the researched territory are represented by deposits of the Permian system (clays, siltstones, mudstones, marls, sands, gypsum, limestones).
Deposits of the Jurassic system (clays, sands, marls, carbonate clays, clay shales) are common on the western, southern and eastern periphery of the catchment basin.
In the extreme south of the basin, deposits of the Cretaceous system (clays, sandy clays, sands) are locally represented [18].
Quaternary deposits in most of the researched territory are represented by middle-upper quaternary and modern deposits of a complex of combined deposits of the periglacial zones of the Dnieper, Moscow and Kalinin glaciations, eluvial-deluvial formations of watersheds, deluvial-solifluction formations of slopes and alluvial-deluvial formations of ancient beams (loam, loess-like loam, buried soils; siltstone and clay sands below, in beams of sand with gravel, pebbles, clay).
On steep slopes, pre-quaternary deposits often come to the surface.On the southern and western periphery of the catchment basin middle-upper quaternary sediments, a complex of combined deposits of the periglacial zones of the Dnieper, Moscow and Kalinin glaciations are locally represented.Floodplains of rivers are composed of modern alluvial deposits (sands at the base with gravel and pebbles, siltstones, loams, clays).
In the north of the basin, upper quaternary deposits of the Mikulinsky and Kalininsky horizons are locally distributed (sands at the base with gravel and pebbles, siltstones, loams, clays) [19].
The catchment area of the Ketarsha River is located in the north of the Privolzhskaya Upland in the central part of the elevated right bank of the Nizhny Novgorod region.The surface is characterized by the alternation of river valleys and interfluves.The general relief is hilly and ridge, the slopes of the valley are dissected by ravines and gullies.The amplitude of heights ranges from 85 meters at the mouth of the Ketarsha River to 226 meters on the western and eastern watersheds [20].
The shallow occurrence of limestone of Permian age predetermined the widespread development of karst, especially on floodplains and floodplain terraces of rivers.
The territory of the catchment basin of the Ketarsha River lies in a temperate climatic zone.
The hydrographic network is represented by rivers, ponds, springs, and shallow marshes on floodplains.
The total length of the channel network of 51 permanent watercourses in the catchment area of the Ketarsha River (as of 2001) was 119.58 km, of which 41 watercourses had the status of elementary watercourses with a total length of 46.7 km.Thus, as of 2001, the total length of elementary watercourses was 39% of the length of all permanent watercourses of the basin.
The soil cover is represented by gray forest soils, chernozems and soils of the ravinebeam complex.On floodplains of riversalluvial-turf soils [21].The landscape structure of the catchment area of the Ketarsha River at different hierarchical levels is reflected in a number of works [22][23][24][25] (Figure 1).
The vegetation of the catchment area of the Ketarsha River has been greatly altered by humans.At present, the structure of the vegetation cover is totally dominated by the agrocenoses of the fields, only on the peripheral watersheds locally preserved oak forests (Figure 2).According to V.V. Alekhin and D.S. Averkiev, in 1926 the situation was the same, however, before the beginning of the active development of agriculture, the indigenous vegetation of this territory was oak forests, in the far north of the catchment basin, turning into the steppe [26].
In the course of field research in 2023 was established the current position of the sources of 22 elementary watercourses out of 41 elementary watercourses, recorded on the topographic map, made in 2001, which is 54% of the total number of elementary watercourses in the catchment area of the Ketarsha River.In 2023 (according to the results of the survey of 22 elementary watercourses), was recorded total reduction in the length of the channel network on 17 elementary watercourses, the total length of which was 23 573 m in 2001, by 11 024 mthat is, the reduction in the length of the channel network of elementary watercourses in the period 2001-2023 was 46,8%.
The total increase in the length of the channel network was recorded on 2 elementary watercourses with a total length (as of 2001) of 335 m and amounted to 335 mthat is, the increase in the length of the channel network of elementary watercourses on these two watercourses in the period 2001-2023 was 100%.
No changes were recorded on 3 watercourses with a total length of 4 357 m.The total dynamics of the length of the channel network of 22 surveyed elementary watercourses was -10 689 m.With a total length of the channel network of these watercourses in 2001 of 26 432 m, the total reduction in the length of the channel network of elementary watercourses over 22 years was 40%.
As it was proved earlier [15][16], a change in the spatial characteristics of some landscape components, in particular, the ratio of arable land, deposits, meadows and forests, is crucial in changing the length of the channel network of elementary watercourses.
The change of land use during the period under review has the following expressions (Figure 3): an increase in the area of arable land due to the plowing of meadows -2.35 km 2 (1.25% of the catchment area of the Ketarsha River).From arable land was transferred to the deposit 11.21 km 2 (5.94% of the catchment area of the Ketarsha River).From arable land was transferred to the meadows 18.79 km 2 (9.96% of the catchment area of the Ketarsha River).4.34 km 2 (2.30% of the catchment area of the Ketarsha River) was transferred from the deposit to arable land.3.48 km 2 (1.84% of the catchment area of the Ketarsha River) passed from the deposit into meadows.0.104 km 2 (0.05% of the catchment area of the Ketarsha River) was transferred from the deposit to the regenerating forests.0.03 km 2 (0.02% of the catchment area of the Ketarsha River) was transferred from arable land to regenerating forests.Thus, the area of arable land in the researched area has decreased by 23.34 km 2 over 22 years, which is 12.5% of the catchment area of the Ketarsha River.

Conclusion
As a result of the conducted research, the fact of reducing the length of the channel network of elementary watercourses in the period from 2001 to 2023 was established.Among the most probable reasons, that determined the direction and quantitative indicators of dynamics, we see climatic indicators, spatial structure of land use; succession of plant communities; change in the nature of land use.Among the most probable reasons that determined the direction and quantitative indicators of dynamics, we see a change in climatic conditions; succession of plant communities; a change in the spatial structure and nature of land use.
The fact of reduction of arable land areas has been established, which is a favorable factor for the development of the channel network of elementary watercourses.However, in the catchment area of the Ketarsha River, degradation of the length of the riverbed network is observed, apparently continuing the trend, laid down several centuries agosince the beginning of deforestation and plowing of land.
Reducing the area of arable land can be considered the first step towards restoring the channel network of elementary watercourses, however, as our research has shown in the Sundovik River catchment area [15], also lying in the northern forest-steppe, the restoration of watercourses occurs only if the source of the watercourse is located in a forest area of at least 5 km 2 .Confirmation of this thesis is that two elementary watercourses in the Ketarsha river basin, on which an increase in the length of the channels is recorded, lie on the edge of a large forest area.
Among the possible measures to prevent further degradation of the riverbed network, it is possible to designate the preservation of the existing few forests, possibly by transferring them to the category of specially protected natural territories, which, in addition to creating favorable conditions for the restoration of elementary watercourses, will give a number of

Fig. 1 .Fig. 2 .
Fig. 1.The position of the territory of the catchment area of the Ketarsha River in the system of landscape zones of the Nizhny Novgorod region.

E3SFig. 3 .
Fig. 3. Spatial dynamics of land use types in the catchment area of the Ketarsha River in the period 2001-2023.