The most suitable place is a (temporary) bridge. Always make the measurement at the upstream side of the bridge! If the bridge has pillars do not make your measurements close to these pillars. The man holding the meter should never be standing in the water. Build a- temporary bridge or use a boat or float instead. Measurements can also be taken upstream of a structure. In any case make sure there are no offtake between the structure and the measurement site. Close them, if another site is not available.