Effect of soil physical, chemical and mineralogical properties on gully morphology in southern marl lands of Guilan province

In this research، morphological characteristics، soil properties، geological and climatical parameters of southern Guilan province gullies were studied. Since، marl lands in south of Guilan province، is favorable bed for gully formation and the source of large amount of sediments in Sefidroud watershed and chief factor for sediment accumulation behind Sefidroud dam، so، study of their properties is indispensable. In this investigation، after geological map observation and land studies، three dominant representative gullies with linear، dendretic and head-dendretic view plan and two repeat of each kind of them was selected and sampled. 36 Samples were carried to laboratory for physical، chemical and mineralogical analysis. The results of these experiments demonstrated that formation of all kinds of gullies is the function of clay content، kind of clay minerals and amount of soluble and exchangeable sodium. So that، with increasing in amount of fine clay and 2:1 smectite clay minerals and Na ions، piping erosion and formation of gullies enhances، as well. This fact، result to branches increasing. This causes that gullies change from linear plan to dendretic one. Also، increasing of clay content and sodium ions from surface to depth، leads to increase in water infiltration to subsurface clayey layer and piping erosion occurs. These processes aggravate gully formation and increase the gully branches.