Paleoclimatic investigation of the Quaternary period reflects fluctuation in climatic conditions during geological periods. Abrupt climatic changes, viewed as problem causing global incidents bring about adverse side effects in sensitive climatic zones such as Iran. Droughts and unpredictable floods that damage natural resources as well as human life become common yearly occurrences. Apart from influences of natural physical factors, human activities, if not properly controlled, add to the destructive power of the floods too. This happened in the Nekaroud basin mainly due to deforestation and damaging changes in landuse during the last century. The huge magnitude flood of summer 1999 which caused irreparable downstream damage and death of citizens is a sad incident of such environmental changes. The present study is an investigation into some characteristics of Nekaroud basin, an analysis of the reasons behind the occurrence of the catastrophic flood, ways to predict and prevent similar disastrous incidents in other catchments basins in Mazandaran as well as in other regions with similar conditions in Iran.

Forests are usually known as systems that can be regulate the quality and quantity behaviors of water resources and the first written researches in this field were done in the Europe, USA and East Africa. This research tried to investigate the role of forests in water conservation and flood decrease through comparison the soil infiltration of forest land uses with the other land uses in order to investigate the effects of these degradations on Neka Rud Forested basin flood in 1999. In this regard, land use maps of this basin in 1967 and 1999 have been prepared through GIS Softwares and their comparison showed that the forest area has been decreased in this period. Afterwards, 10 samples were selected in each land use (high density forest, medium density forest, low density forest, rangeland, and agricultural land) and the soil infiltration in each sample was measured by double rings. According to the results, the highest soil infiltration belongs to high density forest and the lowest one belongs to agricultural land. Also, Duncan Test showed that there is a significant difference between observed data of infiltration in agricultural lands and the other land uses. Horton Formula is suitable for calculating the soil infiltration of different land uses of this basin. Finally, vegetation cover changes have been emphasized as the affective factor on the changes of soil infiltration and flood volume in Neka Rud Basin flood in 1999.