An investigation was conducted to identify the nonpoint sources of suspended matter in the California Colusa Basin Drainage Area, to interpret the factors contributing to or affecting suspended solids loading, and to develop recommendations for best management practices. The 1977-1979 project period (Phase I) focused on an overview appraisal of sediment production and transport. The 1979-81 project period (Phase II) involved intensive quantitative investigations into the causes of the turbidity problem and the development of recommendations for sediment control. The study area included the 70-mile long Colusa Basin Drain (CBD), the 164,000 ac Glenn-Colusa Irrigation District (GCID), and selected tributaries. This report contains data on water flow and suspended and bedload materials and their biological, chemical, physical, and mineralogical properties, rainfall simulation studies to extend the usefulness of the Wischmeier Universal Soil Loss Equation (USLE), a pesticide survey for selected chemicals, and the development and validation of a computer simulation model for sediment transport. This investigation resulted in conclusions and the development of guidelines for best management practices and recommendations for sediment control. These findings, reported in Water Science and Engineering Paper No. 4023, were reviewed by project collaborators and interested parties for feasibility and implementation.