Silt.. sorry missed that question. No, silt does not help or prevent leakage...it is very porous and easily penetrated by water. Unless you are concerned with insufficient water depth, leave it alone would be my recommendation.
If water depth is insufficient, you may be faced with significant problem removing the silt depending on the depth. Search the forum and you will find several discussions on removing silt and/or muck. It is generally cheaper to simply raise your dam height if you need deeper water rather than removing the silt...either way, it isn't cheap or easy.
