Calculate daily means from 5-minute interval data

I have a year's hydraulic data (discharge, stage height, velocity, etc.)
from a USGS monitoring gauge recording values every 5 minutes. The data
files contain 90K-93K lines and plotting all these data would produce a
solid block of color.

What I want are the daily means and standard deviation from these data.

As an occasional R user (depending on project needs) I've no idea what
packages could be applied to these data frames. There likely are multiple
paths to extracting these daily values so summary statistics can be
calculated and plotted. I'd appreciate suggestions on where to start to
learn how I can do this.

TIA,

Rich