computing annual maxima

I have daily streamflow data for a large number of years. One column is the date and the second column is the streamflow value. I want to compute the maximum value and the 3-day maximum (largest average value for any consecutive 3-day period)for each water year (starts on October 1 and ends on Sept 30). Of course the number of days each year varies by one during leap years. Any help would be greatly appreciated!