GC:
The use of a Pump as a Turbine (PAT) in your site is not possible, one of the problems is the head available in your site.
Using your basic GUESSED values :
100 gallon/minute = 6.3 liter/second
head 15 feet = 4.5 meters
The power available is 6.3 * 4.5 * 9.81 = 278 watts, then one needs to apply the efficiencies for each part of the system and for these small power levels the power available would be around 140 watts if carefully done for 3.36 KWH/day of useful harvested energy.
In winter if the water volume is real the energy available would be then about 1.4 KW for 33.6 KWH/day useful, indeed, if a careful analysis of the site and the installation of a well built hydro turbine system.
First step and accurate site evaluation without any GUESSES since water is a very deceiving "animal" that is constantly erroneously "GUESSTIMATED"
Nando