River magic / Ellen Booraem.

Enhanced descriptions from Syndetics:

Magic moves in next door in this hilarious and heartfelt middle grade fantasy about a resourceful girl battling a temperamental thunder wizard.

Donna's always liked her life by the river--that is, until her beloved aunt Annabelle died in a tragic kayaking accident. Now money's tight, her mom works all the time, and her best friend, Rachel, would rather hang out with her basketball teammates than with Donna. When a strange old woman moves in next door and hires Donna to clean part-time, she figures this is the perfect chance to get over her friendship troubles and help her family out--especially since the woman pays in gold. Turns out, Donna's new neighbor is an ancient, ornery thunder mage, and it doesn't take much to make her angry. Before Donna knows it, Rachel is in danger and Donna's family is about to lose their home. To save the day, Donna will need the help of a quirky new friend and the basketball team . . . plus the mysterious, powerful creature lurking in the river.