We have noticed that after about 5 years of No-Till we can fill in the old gullies so they are flat across the side-hill and they will no longer wash out. If the gullies are not filled in then the run-off will flow to them and continue to erode a channel in the middle.
I've noticed some interesting erosion/silting patterns. If the erosion pattern is oval on the side-hill the silted in pattern is oval at the base of the hill. Water creates a similar pattern of eroding/silting.
Terraces help but also give a false sense of security to till. We have found that terraces are needed on long slopes and/or D & E slopes even with No-Till. We have had a record number of floods/storms in the 90s and the fields that were in No-Till since the early 80s and had terraces withstood the 1993 11 inch rainstorm. All the tillers had terraces blow out but our terraces weren't even topped by this 500 year rainstorm.