At the catchment scale:
Stream order, stream length, distance to source, distance to downstream main confluence, distance to the closest flow recorder, number of active flow recorders in the catchment, catchment areas, climate zone (Koppen system, http://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification), landscape uses (forest, agricultural lands….), % of the network being intermittent (estimations), average annual precipitation.
At the reach scale:
Active channel width, width of the floodplain when relevant, estimate the % cover of the following substrate types (silt/sand/gravel/cobble/boulder/bedrock) across the reach, riparian cover (estimated visually as a %), riparian vegetation (absent/herbs/shrubs/trees), the 3 dominant riparian species, estimates of the drying period duration and timing, type of drying (flow cessation with dried riffles and persistent disconnected pools, or complete drying), X, Y (in WSG84) and altitude (m asl).
Take a few pictures of the study reach.