In the early days of development, in an embryo, the cells are pluripotent and can differentiate into any kind of cell of the organism. As the embryo develops, autocrine and paracrine signaling between the cells of the embryo causes the cells to migrate to particular regions of the embryo and begin differentiating into respective parts of the organism. This signaling causes differential silencing of particular genes of the cells depending on the type of cells they will differentiate into. As the embryo develops into a fully grown organism, these cells lose potency (except for stem cells) and become fully differentiated into respective limbs or organs of the organism.
A river that comes from rain, glaicers etc would be a good example of fresh water rivers. Cause the water is coming from something that is most likely been unthouched by man or has been filterd and cleaned from minerals in the ground.
