Typically, the sides of streams and marshes contain various soil types because of the different moisture levels in their steep or gradual grades. For this reason, along long stretches of the lake trail, plants of dry soils, like this native milkweed, grow only feet away from hydric species like wild iris.
This very simple topographical feature is one of the factors which create such biodiversity in riparian (stream-side) habitat, and one of the reasons for the importance of their preservation.