I can't sleep, so I looked up UC-Riverside's website. Going on the assumption that you are interested in rushing an NPC sorority, your choices are:
Alpha Chi Omega
Delta Gamma
Gamma Phi Beta
Kappa Kappa Gamma
Pi Beta Phi
Sigma Kappa
Go here and then click onto each sorority's link. It will take you to their National websites so you can learn a little about each sorority:
http://www.studentlife.ucr.edu/greek/organizations.html
Oddly enough, there is no mention of rush dates, but there is a phone number you can call for information...
UCR Student Life & Leadership Center, 145 Costo Hall. (909) 787-7344.
Good Luck!!!