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Time of day for lowest water levels at creek crossings
#50279 06/22/17 01:13 PM
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Syoung's excellent recent report and photos from his day trip up the hikers trail certainly gave me pause about water levels at creek crossings. (I recommend checking it out if you haven't yet.) I'm planning to go up the NF of Lone Pine Creek the weekend before July 4 and spend a few nights at Iceberg Lake. My intended route and syoung's are different, but since they're in the same general vicinity I assume water levels at creek crossings will be similar. I had been contemplating start from Whitney Portal at around 4-5am in order to avoid high water flow at creek crossings and to minimize post holing. However, seeing syoung's photos of the sketchy snow bridges over raging waters in the pre-dawn darkness gave me pause. Would the water levels be significantly higher if I started 1-2 hours later from WP, at first light? What do people think is the time of the day/night when water levels are at their lowest? I get that they're likely at their worst/highest in the afternoons after the day time melt. But it's not clear to me that 4am vs 6am would make a big difference. Any thoughts? Thanks in advance for everyone's collective wisdom!

Last edited by Fan; 06/22/17 01:31 PM.
Re: Time of day for lowest water levels at creek crossings
Fan #50337 06/26/17 07:00 AM
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Bump, in case someone has advice.

Re: Time of day for lowest water levels at creek crossings
Fan #50339 06/26/17 08:25 AM
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I have no real data here, but I think there would be a slight change in flows between peak and low, and probably occurring a few hours after the peak and low temperatures for any day. However, I doubt the flows would change significantly enough to make it worthwhile to try to time your crossings.

If you are going up the North Fork, the best way to avoid the stream crossings is to go up the Rockwell shortcut, which stays on the north side between the first and second stream crossings, below the Ebersbacher Ledges. That shortcut requires a short section of third class climbing up a broken crack in the granite, and then walking a narrow ledge that can be more intimidating than the Ebersbacher ledges.

Re: Time of day for lowest water levels at creek crossings
Steve C #50340 06/26/17 08:39 AM
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Not sure you can see this picture, but Sequoia N.P. posted on their FB page this picture, saying the difference is between 7 AM and 7 PM. So it appears there IS quite a difference in flow rates.


Re: Time of day for lowest water levels at creek crossings
Steve C #50346 06/26/17 11:42 AM
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Thanks, Steve! That is a huge difference between 7am and 7pm. I think I'll try to leave WP trailhead just early enough to arrive at the first crossing with sufficient natural light. Depending on how the first crossing goes, I'll either check out the second one or head straight for the Rockwell Shortcut.
I found your photos for that detour to be very helpful.

Re: Time of day for lowest water levels at creek crossings
Fan #50350 06/27/17 07:41 AM
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Originally Posted By: Fan
... Would the water levels be significantly higher if I started 1-2 hours later from WP, at first light? What do people think is the time of the day/night when water levels are at their lowest? I get that they're likely at their worst/highest in the afternoons after the day time melt. But it's not clear to me that 4am vs 6am would make a big difference. Any thoughts? Thanks in advance for everyone's collective wisdom!

Think about the physics of this problem and Hydrology 101. There is a slight delay between the peak sunshine at noon, the peak temperature in mid afternoon, and peak flow from snowmelt because it takes time for the snow to warm and melt water to percolate through the snowpack. The snowmelt delay is a minor delay, and a lot of the melt occurs around the edges and bottom of the snowpack where it doesn't need to percolate anyway. So peak melt is sometime in the afternoon, with several hours of high temps this time of year. The next delay is for the water to accumulate in tributaries and converge into main stream/creek/river. This "time of concentration" effect can be the most significant and depends on the size of the watershed, terrain surface, and slope. Whitney area here is very steep and rocky so this delay is probably just a few hours, maybe 1-3? The other significant delay is the time it takes for the water to flow down the main creek to your particular crossing, which depends on the distance from the snowpack and slope of the creek/river. We're pretty close to the melt here, so that's maybe an hour. I'm thinking 2-4 hrs is a good guess, which puts the peak at around 5-7pm roughly. Caveat: if there is a lake or lakes between the snowpack and creek crossing, the reservoir(s) will attenuate the peak and create another delay in arrival time downstream, perhaps another hour or few (depending on size of the lake).

In any case, the "peak" for snowmelt is broad curve that changes slowly over hours. This is very different from a typical large rainfall event where the peak is usually a large spike in flow that rises and falls quickly and then tapers off gradually.

For the low flow that you've asked about, the snowpack cools all evening and may refreeze. Low temp is around 4-5am, and minimum flow might be around 5am-8am and stay low until the snow starts to melt seriously in mid-morning and work its way down to the crossing.

Alas, for this situation at high elevation, steep slopes, and rocky terrain paths, I think you'll see very little difference in creek flow between 4am and 6am, but you'll have daylight at 6am. How's that for a convoluted answer?

Last edited by SierraNevada; 06/27/17 07:57 AM.
Re: Time of day for lowest water levels at creek crossings
SierraNevada #50356 06/27/17 09:28 AM
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SierraNevada, thank you for the fantastically knowledgeable, well-reasoned, and on-point response! I really appreciated understanding your logic and theory in arriving at an answer to my question and your accounting for the particulars of the situation here. Given little difference between water levels at 4am vs 6am, I think I'll probably depart from WP closer to 6am than 4am in order to take advantage of the daylight. Of course, that means slightly warmer temps and softer snow near the end of my approach to Icerberg Lake, but life is full of trade-offs.

Re: Time of day for lowest water levels at creek crossings
SierraNevada #50362 06/27/17 10:30 AM
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Dam engineer. wink

Re: Time of day for lowest water levels at creek crossings
Steve C #50372 06/27/17 12:45 PM
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Shown below is data for Lyle Fork 9650' in Yosemite backcountry, somewhat like certain areas in the Whitney area but this watershed is pretty large and there are lakes upstream. There are very few High Sierra gauges out there for streamflow and this one is currently out of service.

Data plotted is from late June 2014 (a dry year).

Note stream stage fluctuated daily about 1.2' The peak occurred around noon, leveled off all afternoon then dropped fast starting around 6pm. Low flow and best time to cross was between 4am and 8am.

This example illustrates the general trend for high elevation creek/river crossings. As you move downstream to lower elevation things change and the peak can occur in the evening and best time to cross might be in the afternoon.



Re: Time of day for lowest water levels at creek crossings
SierraNevada #50376 06/27/17 01:03 PM
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For comparison, the plot below shows the West Walker River near Coleville & HW395 at 6700 ft elevation and much further downstream in the watershed. Note the peak stage here occurs in the evening and the lowest stage is in the afternoon.

These examples illustrate how much things can vary within a large watershed and hopefully provides some insight into what is happening and why. At lower elevations with lakes and many tributaries, the best time to cross might be in the afternoon instead of the early morning. If you're camped near a river you're worried about crossing, you can set a marker to see if the flow is increasing or decreasing and how fast its rising or falling to make your best decision. With a record runoff year like 2017, the wait may be worth it.


Re: Time of day for lowest water levels at creek crossings
SierraNevada #50378 06/27/17 01:16 PM
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this is great stuff.

One of my most memorable learning experiences on a hike was with a geologist. Next time I need a hydrologist/engineer. Thanks SN

Re: Time of day for lowest water levels at creek crossings
Harvey Lankford #50380 06/27/17 02:20 PM
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Agree with Harvey. Great stuff. Thanks again.


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