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So, my wife and I were hiking today and spent some time along the Poudre River here in Colorado when I came across an odd rock formation. First, because of the drought conditions, the very low snow pack, and the pretty gnarly fires we had the levels of the Poudre River are extremely low. So low, in fact, that there are quite a few things that have been exposed. Anyway, this odd rock formation doesn't look entirely natural. It struck me that it reminds me of a rock Fish Trap. Here is one for comparison:

http://forwhattheywereweare.wordpress.com/2010/10/09/epipaleolithic-fish-traps-from-wales-and-ireland/

And here is what I saw today. So, is this a fish trap? It looks to be filled with a lot of loose sediment.

Does it look like there ever might have been a side stream that flowed into it? Otherwise it's facing the wrong way by 90 degrees to trap fish from the current riverbed. It needs to be parallel to the riverbed not perpendicular to trap fish:

Woo ASCII Art fish trap:
______
~~~F~~\
~~~~F~|
~~~~F~/
-------/

I see what your saying. It wouldn't be a fish trap, per se. I think I was referring to a more artificial tide pool type of thing. It definitately isn't like a weir or most fish traps. Maybe for crayfish or freshwater clams? Because of the dams on the river, it doesn't flood and then recede like it used to. It just looks man-made.

To me, that looks like some kids playing in the river. And by "kids" I mean anything from a couple 12yo boys trying to build a swimming pool, to a group of college students having a covert party and needing to chill the beer without having it wash away.

Ludi, I think you just answered the OP's question.

I agree with ludi, having built several "beer traps" myself. I would not expect something built years ago to be intact on a river bottom. One simple minor flood --- gone.

Looks natural to me...

Imagine a larger rock falls into the river... causing a zero velocity point in a line roughly parallel to the stream direction. Overtime all the surrounding rocks and sand are made smaller or moved on by the higher velocity stream, but the path with a lower velocity due to the larger rock in the way causes all those smaller rocks behind it to not get eroded away as quickly...

Hence arriving at the structure you see now.

Resembles a pool to keep geothermal water from mixing too quickly with stream water. People who like to hot tub in natures bounty will build a circle of stones around a warm seep. The diameter and depth are typically related to the flow strength and temperature of the incoming geothermal water. Make the circle too large and the pool is too cool. I've seen some pools barely large enough for a single person to soak.

Points taken. However, here is where the waterline usually is:



And here is a side-shot of the distance between the shoreline and the current waterline.



And if you l ook at the sediment buildup, it isn't the blackened ash that we see everywhere else. Seems like in a typical year, this would be pretty deep and far from the shore for a beer cooler.

As for the geothermal aspect, I did put my hand it in, but I wasn't really looking at the temperature. Cool bit of knowledge to pick up though.

I think it's amazing to see rocks in the river..I'm in the south, and we have nothing but sand or mud shore lines. The only time we see rocks is on a jetty and the were put there by man.

ludi wrote:

To me, that looks like some kids playing in the river. And by "kids" I mean anything from a couple 12yo boys trying to build a swimming pool, to a group of college students having a covert party and needing to chill the beer without having it wash away.

TheEmrys wrote:

Points taken. However, here is where the waterline usually is:

Yeah, but it's not like we were only in a drought for the last 3 days. Other hikers probably were in the water while it was super-low and built the booze containment zone. Maybe even to cool some bottled water. Look at where the waterline has been in the past. Do you really think that little formation of stones would last weeks or even days with that increased depth and flow rate? Next time the mountains melt and the area's full of water, go back to the area and see what I mean.

I changed my mind, this video explains it: http://www.youtube.com/watch?v=PLVjt4PNtAE