Friday, March 7, 2014

How to flush a hippo pool

I know our last post was several weeks ago, so you all probably think we have been off celebrating ever since getting our experimental streams up and running... Well, we did do some celebrating, but we've also been super busy with a lot of other cool science work. Having the streams work well just opened up a lot of time for us to do other things. 

One of the things we have been particularly excited about it trying to re-create the dissolved oxygen (DO) crashes we have seen in the Mara. We have developed the hypothesis that it is the flushing of a lot of water through hippo pools that causes these events, but how can you test that experimentally? If we could flush a lot of water through a hippo pool without adding or changing anything else, and if that movement of water alone caused a DO crash, that would be pretty convincing, right? But how would one pull that off?

We toyed around with hiring a huge water truck to dump a bunch of water into a hippo pool, but there were some logistical challenges to consider. In running this idea by our hyena researcher colleagues, David Green suggested, "Why don't you just build a small dam and then remove it quickly?" 

Brilliant! Six tons of sand later and we were in business!

This is what 6 tons of sand looks like
First we found a narrow reach of small tributary we thought we could reasonably block off. Then we just started piling up sandbags.

David showing off his dam-building skills
 After several hours of work, we had constructed a pretty good dam! It was pretty funny to watch Emma and David working on this dam, because they have both done research on the harmful effects dams can have on river ecosystems, but knowing it was temporary made a big difference!

Our completed dam
 We left the dam overnight so it could back up as much water as possible. By the next morning, it had built up almost a 1 foot head, which reach back at least 100 m. That's a lot of water!

Our dam the following morning
After taking lots of measurements and getting our meters spread out downstream, we pulled all the sandbags out of the middle of the dam quickly, letting the water rush through. I was amazed by how big a wall of water we had managed to build up!

Breaching the dam
Chris will be presenting on this experiment and what we learned from it at the Joint Aquatic Sciences Meeting this May, so stay tuned to learn more about our results. Did we re-create a DO crash, or just build a lot of muscles moving six tons of sand around? I'm going to leave you with a little bit of suspense... but I will say that, 24 hours after we started, the river was completely back to normal, but we had learned so much about how these events work! Thanks to David Green for the great idea, and to David, Emma, Chris and the sand guys from Talek for all the hard work!

24 hours later

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