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O[J"t66:' 30.5--3 '." WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK ~omoroholQKY. That branch of both physiography and geology which deals with the form of the earth, the general configura- tion of its surface, and the changes that take place in the evolution of landforms. HYdro~raoh. A graph showing stage, discharge, velocity, or other properties of water flow with respect to time. When the discharge is shown against time, the graph is a discharge hydrograph, or commonly called simply a hydrograph. ~r_~rm. Equivalent to the "depositional-bar level" described as a longitudinal, in-channel feature formed along the borders of a stream channel at a stage of the flow regime when the local competence of the stream is incapable of moving the sediment particles on the submerged surface of the bar. Non-Erod1~le Channels. Those stream channels or channel reaches in which the material forming the bed and banks determines the morphology of the channel rather than the force .of moving water. Perennial Stream or Reach. A stream channel or channel reach having measurable surface discharge more than 80~ of the time. Pore Pressure. The pressure created in the void spaces between soil particles by air and/or water. When a lake shore or stream bank is submerged, the pressure in the pores of the soil mass is equal to the hydrostatic head due to the water elevation (ignoring any velocity effects). A rapid drop in water surface elevation results in a lack of the counter-balancing pressure of the water body. Since the pores drain slowly, the pore pressure is positive (greater) relative to atmospheric pressure and thus may expand the pores and move the soil mass. Recession. The third of three parts of a typical single-peak simple hydrograph. It represents the portion which descends from the peak (the falling or lowering segment). The lower portion of the recession segment is a groundwater recession (or depletion) curve which represents the decreasing rate of groundwater inflow. Recurrence Inter~. The average interval of time, in years, wi thin which the magnitude of an event will be equaled or exceeded once. Reoresentative Station. A gage station used to develop a normalized flow-duration curve for use on hydrologically similar ungaged watersheds. It is the one station, among those available, that is most similar to the ungaged point(s) of quantification. *-FSH 4/89 AMEND 3-*