When the discharge of a river increases, the channel may become completely full. Any discharge above this level will result in the river overflowing its banks and causing a flood. The stage at which the river will overflow its banks is called bankfull stage or flood stage. For example, the flood stage of the Mississippi River at New Orleans 17 feet. Discharge is not linearly related to stage because discharge depends on both the depth and width of the stream channel, or more precisely, on the cross-sectional shape of the channel.
Stage refers only to the height of the water above some reference level.
For example, the graph below is a hydrograph of the Mississippi River at St. Louis, Missouri during the time period of the 1993 flood. The sudden drops seen in discharge around July 15 and July 20 corresponded to breaks in the levee system upstream from St. Louis that caused water to flow onto the floodplain upstream, thus reducing both the stage and discharge measured at St. Louis. To illustrate, for the Mississippi River flood at St. Louis, idealized cross sections of the River are shown below for points a, b, and c in the diagram above. Affect Flooding As discussed previously the main factors that cause flooding are heavy rainfall, sudden or heavy snow melt, and dam failure. Now that we understand something about levees and floodplains, we can add to this list the possibility of levee failure.