In the post-glacial period, a flood of melt water was walking through the vast plains of Eurasia and North America. Later, more calmly current broad river only during high water occupied the entire valley and the original left there brought precipitation. So formed on mineral soils, recharging groundwater, which eventually formed coastal wetlands and floodplain forests. On the bank of the river flowing into the sea in shallow waters set aside brought their stuff. On the fertile clay soils resulted from shoaling first salt meadows, and after washing out of salt - coastal wetlands. In forest-rich areas as a result of lightning strikes and fires appeared clearings, there are beavers create dams in rivers and streams. Difficulties sometimes large reservoirs resulted in the death of flooded forests. When time began to miss eating beavers or their number decreased due to disease, then came the breakthrough of dams. At the same bottom of the lake grass thrives. Through these natural meadows river cut a new channel itself. Often, forest renewal was delayed due to destruction of seedlings by wild animals. These grasslands provide, for example, in North America, the first European settlers to the rich harvests of hay or fertile soil for farming, and in general have long been wet; soil devoid of woody vegetation is much easier for people to agricultural use.