This is a very sad story.
Riverside closed in 1988, when the land was bought by real estate developers.
In brief words, the Northernmost area is today part of a shopping mall parkling lot. There is a housing development (sprawl) that occupies a part of the mid-section of the track.
The rest (actually most of it) is all abandoned. I have been there recently and the there is even less remains that those shown in the (very good) http://www.tamsoldracecarsite.net/RiversideToday.html
Amongst all former circuits I visited - Chimay (B), Rheims (F), Piracicaba (BR), Santa Monica (USA), Palm Springs (USA), Bremgarten (CH), Montreux (CH), etc. - this is the one that really makes my heart break.
No kidding, I almost cried when I saw it.
P.S.: I posted a thread today about Bridgehampton, a road course in Long Island - used to be a Can-Am and sportscar classic - that succumbed a similar death in 1997. Any information about Bridgehampton would be greatly appreciated. Help to preserve the memory of motorsport.