Find and plot local minimums

I have water height data of a local stream over time. The issue is that this stream is affected by the tides. I am wanting to remove the daily tide peaks and just get that minimum water height base line (blue) which hovers around 0.3 meters (right y-axis). Any suggestions would be appreciated!

Can you please post a sample of your data (see here) and any code that you've used to generate the plots?