The trend of geometric changes in Qarachai River in the years 1968 to 2021

Introduction
Due to river dynamic nature, the morphological characteristics of rivers are always changing and these changes can have negative effects on structures built along rivers, agricultural lands, etc. Rivers have been one of the most important factors in the geomorphological processes of the land and the erosion cycle, and so far, extensive studies have been conducted on the morphological changes of rivers and the factors affecting them. The instability of the river channel and its sediments not only destroys the marginal lands and facilities adjacent to the river bank, but also the sediments caused by erosion in some cases include a significant amount of the total sediments that the river flows.
Materials and Methods
The morphology of a part of Qarachai river in Markazi province of Khondab city from the top of Shaveh village to the bottom of Farab village, with a length of 12 km has been studied. First, the location of the Qarachai River watershed and the study area have been determined using digital topographic maps and ArcGIS10.2 software. Then satellite images and aerial photographs of the study area, which are the most important tools for studying changes in river paths, have been collected in two time periods. In the following, the aerial photos scanned in Photoshop software are photomosaic and are referenced in ArcGIS10.2 software using fixed points such as villages, bridges, etc., using aerial photographs of the year. 1968 Satellite images of Landsat sensor of 2021 have been digitized in two time periods in ArcGIS10.2 software environment.
 Results and Discussion
Changes in wavelength and valley length in the two time periods studied in the study section of the river show that in the study area, the morphology of the river has not undergone major changes and the curvature of the study area has not changed much. The study of changes in wavelength and valley length and arc length in the two periods studied shows that the average wavelength in 1968 is about 413.25 and in 2021 is about 387.63. The wavelength in 1968was longer than in 2021. The changes compared to the 53-year period are about 6.2% and very small. As the wavelength decreases, the distance between the windings also decreases. The changes in the length of the valley relative to the meanders. The average length of the valley in 1968 is about 224.46 and in 2021 is about 12.21. Changes in the two timelines are about 5.5% and are very small.
Conclusion
The general pattern of the river from 1968 to 2021 has not changed significantly and is always twisting. The number of windings during 53 years is the same as 23 windings. The fact that the number of meanders is constant indicates that there is no change in the morphological pattern of the river. The results of extraction and statistical comparison of geometric parameters of this part of the river such as wavelength, sine coefficient, width to depth ratio, etc. show that the sine coefficient of the river from 1.5 to 1.3 The wavelength and radius of the tangent circles on the twists and turns have also increased. The average curvature coefficient in the 40s is about 1.11 and in 2021 is about 1.08. According to the division table of rivers, this period studied by Khondab river has a sinusoidal state and has not changed much in two time periods.