Understanding Timing of Merced River Discharge

[MichaelRPetrick]

Here is the last week of data from the Merced at Happy Islses:



Unless I'm much mistaken, the flow peaks a little after midnight, and is at it's nadir late in the day.

Why would this be the case?

Or am I just misreading it?

Source: https://ca.water.usgs.gov/webcams/happyisles/

[Jimr]

I'm not convinced those are midnight to midnight measurements. Noon to noon seem to make more sense. Interesting that it says last gauge reading was at 1:30pm which would imply the lines being midnight to midnight.

[zacjust32]

Could be that way because of the delay between snow melt and runoff. If the snow melts at 10k at noon, it takes time for the water to get downstream. Similar to how June 20th is the longest day of the year, but not the hottest; there is a delay between the discharge and the peak.

[Dave_Ayers]

The vertical bars are midnight to midnight.

The lag has to do with the distance from the gaging station to where the snow is melting. In this case, the station is a number of miles from the main snow pack areas along the canyons surrounding the upper Merced. It takes hours for the snow to melt in the peak sun in the afternoon, trickle through snow packs, collect in streams, and eventually flow all the way down to Happy Isles where the gage shown above is located.You'll see a similar effect for other stream gages. Those gages close to the snowpack will peak in the evening. Those further downstream peak later. See for instanct the T and M together on one graph at https://waterdata.usgs.gov/ca/nwis/uv?s ... 5&period=7 . The T peaks a few hours earlier than the M due to the distances involved. (Jim, if you point at that graph you will see the time pop up too.)

Some even have a double peak where two drainages flow together. See for instance the lower Moke at https://waterdata.usgs.gov/ca/nwis/uv/? ... 0065,00060 .

[Jimr]

Well, now I'm convinced that it is Midnight to Midnight.

https://waterdata.usgs.gov/ca/nwis/uv?c ... 2017-06-28

David beat me to it.

[Jimr]

(Jim, if you point at that graph you will see the time pop up too.)

I was initially looking for the underlying data, but got caught up and frustrated, so I narrowed the scope and time popped up.

[TahoeJeff]

The lag has to do with the distance from the gaging station to where the snow is melting.

Same thing on the EFC:

[MichaelRPetrick]

Thanks for the replies all - especially graph of the Moke. Helps me get the big picture.