The plate shows a graph of the maximum and minimum monthly mean flow and mean monthly river flow for 36 selected river gauging stations. Thirty-six gauging stations across Canada were selected to represent a cross section of river types and sizes. Most of the rivers are natural flow; however, minor regulation does occur on some for various reasons, such as hydro development, storage for dry periods, irrigation, and flood prevention. The shape of each plot shown on the map depends to a large extent on the amount of storage in each basin. For example, the Saskatchewan River basin has comparatively little storage and a sharp peak and dip, while the Churchill River basin has a large amount of natural lake storage and a smoother curve. The large storage tends to level out flows and reduce the predominance of the peak flow in the river. A good example of this effect is the St. Lawrence River where, because of the very large storage of the Great Lakes, little variation is evident.