# Don't let your guard down



## lmyers (Jun 10, 2008)

I saw a bunch of flips on the 4th, if you look at the private boater photos from AVDI it was nearly 1 in 4....BUT, that was at 2200, 1700 this morning and still dropping. Definitely getting more friendly all the time. Stay safe and have fun out there!


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## jaffy (Feb 4, 2004)

Out of curiosity, when you say it's a 20 year high for colorado, does anyone know how they calculate that? I know the Poudre peaked higher and for longer last year, so I'm assuming other rivers had higher flows this year that more than made up for it. Do they take the peak flows of all the rivers and add them together, add up the total volume of water that flowed down them over the season, or is there some other metric?


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## bdf48 (Mar 4, 2010)

jaffy said:


> Out of curiosity, when you say it's a 20 year high for colorado, does anyone know how they calculate that? I know the Poudre peaked higher and for longer last year, so I'm assuming other rivers had higher flows this year that more than made up for it. Do they take the peak flows of all the rivers and add them together, add up the total volume of water that flowed down them over the season, or is there some other metric?


It is different for every river (or even sections of river). To calculate you take the annual peak for each year there is a record. If there are 100 records than the largest is the 100 year flow... then 20th largest is the 20 year flow. If there are less records than you extrapolate from the data that you have using stats.


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## Jones (Apr 5, 2005)

*flow frequencies*



bdf48 said:


> It is different for every river (or even sections of river). To calculate you take the annual peak for each year there is a record. If there are 100 records than the largest is the 100 year flow... then 20th largest is the 20 year flow. If there are less records than you extrapolate from the data that you have using stats.



Your on the right track, but not quite right in how you are calculating the 20 year flow. It all comes down to statistics. On average a 20 year flow would happen once every 20 year. Or, to look at it another way, a 20 year storm (on average) would happen 5 times in a 100 year period. From that, you can estimate the the lower limit (or cutoff flow) of the 20 year flow based on the flow rate of the 5th highest flow from the 100 year period. This works because all flows above this are (on average) at least a 20 year storm. Because the flow labels come from statistics, it is possible to have two 20 year flows in a row.

Above was a very quick, and brief idea of how the frequency of large flows are determined. A much better explanation can be found here:

Floods: Recurrence intervals and 100-year floods


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## DoStep (Jun 26, 2012)

Easier yet, the last time we had these sustained levels on the Upper Ark was in '95, or 20 years ago. Without looking at the data, I recall the time before that was in the 70's, or about 20 years prior. But that was before I discovered boating, so who cares? 

The USGS site has some good data searching tools, check it out.


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## jaffy (Feb 4, 2004)

My mistake, I thought you were saying it was a 20 year peak flow for the state of Colorado, and I was trying to understand how you would even calculate that. River by river is easy, and it sounds like this was for the Ark.


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## lmyers (Jun 10, 2008)

DoStep said:


> Easier yet, the last time we had these sustained levels on the Upper Ark was in '95, or 20 years ago. Without looking at the data, I recall the time before that was in the 70's, or about 20 years prior. But that was before I discovered boating, so who cares?
> 
> The USGS site has some good data searching tools, check it out.


I'm pretty sure we had bigger sustained flows in 2011. Every time I have looked at the below Granite gauge in the last 2 weeks it has shown that it was higher in 2011...for example right now it says it was still 2730 in Numbers on July 7th in 2011...


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