silt 
(sĭlt)
(sĭlt)n.
A sedimentary material consisting of very fine particles intermediate in size between sand and clay.
v. silt·ed, silt·ing, silts
v.intr.
To become filled with silt: an old channel that silted up.
v.tr.
To fill, cover, or obstruct with silt: River sediments gradually silted the harbor.
[Middle English sylt, probably of Scandinavian origin; see sal- in the Appendix of Indo-European roots.]
silt·ation n.
silty adj.