Assessments are nothing more than tax bonds that the County of Riverside allowed the builders to pass onto the homeowner. The bonds are used for infrastructure and other street and community improvements. They have a time limit of 20 or 30 years usually.
You can find out more at the Riverside County Tax Assessor's website: http://riverside.asrclkrec.com/
Or go to one of the Riverside County Tax Assessor locations to ask about a specific property.