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How would the Hoh river, glacier fed, have peak flow at about 9am?
The meter is about 20 nautical miles from the source, and Google suggests it's about 40 river miles. It should only take 10 hours for that peak melt flow to reach the sensor: when I float in the mid section of the river I with no wind, I drift at an average about 4 mph.
The main reason for the peak flow to happen at a certain time is that the meltwater pulse takes that amount of time to propagate.
If the peak Hoh River flow 40 miles from the source happens at 09:00, one could assume the the flow matches the 4mph that you observed, or that peak melt happens around noon the day before.
At 4mph, then the melt pulse at the source would happen at 09:00 - 40mi/4mph = -01:00 or 23:00 the night before. From 12:00 the day before would make the peak take 21 hours to travel the 40 miles, or about 2 mph.
That differs from the 4mph you observed while floating the mid section.
One explanation could be that the surface water you floated in moves faster than the average flow in the river. That is a common feature what is called "Open channel flow" in fluid dynamics, since the velocity profile approaches zero as you approach to the bottom--the average flow is less than the surface flow.
