It is Joni Mitchell.  She was one of the other acts, along with Jessee Colin Young & the Beach Boys.  This show was at Roosevelt Raceway on the day that Ford pardoned Nixon (a Sunday if I remember correctly).  They also played at Roosevelt Stadium in Jersey City on the night that Nixon announced his resignation.  At least I can still remember where I was on those 2 dates.  I still have the tee shirt I got at the Roosevelt Stadium concert.  Not that I can fit into it.