Contact filtration (deep-bed filtration of lake water after particle destabilization but no other treatment) was studied in laboratory-scale experiments. Particle size distributions and pressure were measured at four different depths for several hours, the time over which filter performance generally improves, i.e., ripening occurs. Particle size distribution measurements coincided with measurements of turbidity and suspended solids but provided more detailed information. Ripening occurred for all sizes initially and continued for small sizes for several hours, but it slowed or reversed for larger particles. Increased media size (from 0.8 to 1.85 mm) allowed use of the entire bed for removal and reduced head loss, although greater depth was required for equivalent removal. Filtration velocity also affected removal efficiency, bed depth use, and head loss--but to a lesser extent than media size. Includes 41 references, table, figures.