Fresh water swamps may be saturated part of the year and quite dry part of the year. They are fed primarily by surface or ground water and may be tidal along rivers in the coastal plain. They are often hummocky (semi dry islands anywhere from a foot to a few yards wide) with shrubs and trees. The types of trees vary with latitude. Northern white cedar, larch, black ash and alder may dominate in the northern eastern part of the country. Water oak, ash, tupelo, and bald cypress may grow further south. In the northwestern part of the country willow and red alder may dominate.