Calibrating Manning's N in HEC-RAS

Hi:

I'm currently working on a fisheries habitat project in which I need to determine water surface elevations in a stream over a range of discharges. Data I have collected so far includes: stream cross-sections, stream profile elevations (ie longitudinal profile), and water surface elevation at a single low flow. How do you determine Manning's n using HEC-RAS at the low flow, so that it may subsequently be used to predict water elevations at higher flows. I am aware that Manning's n will change with discharge, but am assuming it is constant for now.

Thanks. Any comments would be greatly appreciated.
Paul Godman