ChannelArea(Distances, Elevations, Stage)
Calculates cross sectional area of a channel at input stage.
Input:       Distances = Horizontal distances across the channel (numbers)                   Elevations = Channel bed elevations at each horizontal location (numbers)                 
Stage = Stage at which to calculate the channel area (number)
Output:      Channel area at input stage, in same units as distances, elevations, and stage (number) |