I wanted to draw your attention to Sasha Natapoff's recent post on Prawfsblawg about prison rape. She links to a two-part essay in the New York Review of Books about the variation in state efforts to stop prison rape. I really recommend the post.
There is also a California angle: UC Irvine researchers at the Center for Evidence-Based Corrections prepared a 2007 report to CDCR about sexual assault in CA institutions. Conducting face-to-face interviews with 322 randomly-selected inmates, Val Jenness, et al., found great variance in the frequency of assaults: 4% of the interviewee sample reported being sexually assaulted in a CA institution, but the rates were much higher among special populations: 59% of transgender inmates reported abuse. While sexual assault was prevalent for almost all inmates, non-heterosexual inmates and Black inmates were considerable more vulnerable to sexual assault.. There was also a strong correlation between victimization and mental health problems. The report includes some details about the characteristics of incidents, notably the gang context of many incidents, and the difficulty in labeling incidents as coercive, as opposed to "part of prison life", even when all incidents invoke fear, self-blame, and subsequent problems. Jenness et al also address overcrowding as a contributing factor to sexual assault incidents.