What does the letter ‘r' represent in the s. p.i. d.e. r. method?

The color contrast between the open ocean and the lagoon between the spit and the mainland. the greater concentration of suspended sediments in the lagoon gives the waters a more turbid appearance. the source of sediment in the lagoon is (are) [(rivers and streams from the mainland) (wind-borne sediment from the spit) (ocean waves washing over the spit) (all of these contribute)].