Water Biomes Marshland is covered with grasses, reeds, sedges, and cattails. These plants all have their roots in soil covered or saturated with water and its leaves held above water.Marshes may be freshwater or salt. Freshwater marshes develop along the shallow edges of lakes and slow-moving rivers, forming when ponds and lakes become filled with sediment. Salt marshes occur on coastal tidal flats. Inland salt marshes occupy the edges of lakes. They affect the supply of nutrients, the movement of water, and the type and deposition of sediment. Salt marshes are best developed on the Atlantic coasts of North America and Europe.