The Riverside County Sheriff's Department has been ordered to reduce the crowded county jail's population by about 100 inmates by March and by another 200 inmates by July, 1989. The order, which sets population limits for all county jail facilities, was signed by Howard M. Dabney, a former Superior Court judge who last month was elevated to the 4th District Court in San Bernardino. It requires the Sheriff's Department to have an inmate-reduction plan ready by Feb. 1. Crowding at the county jail has been so severe at times that inmates have been forced to sleep on mattresses on bathroom floors. Chief Deputy Larry Smith said a number of measures are being considered, including alternatives to incarceration such as closely supervised probation. A 752-bed jail is under construction in downtown Riverside and should be completed in May, 1989.