When do they drain the river??

I think every year they find out if things in the water bones locks and everything

Not sure. I think it is every 3 to 4 years.

This website might be of some help to you. I honestly thought it's 1-2 times a year but in different sections of the river. Take a look at this: https://www.sanantonio.gov/CCDO/riverwalk/maintenance

The boat guide said every year in january after the holidays when things get relatively slow.