Tolerant Weld. In radiants menu there is TW. Make sure TW is selected.

The best way I've found TW to work. Set your grid size to 4 (if your not careful, your grid size is too large. You can end up with multiple vertices joining in one location).

Select a few terrain patches where the breaks are. Once selected, press V to show the vertices. Then just drag a huge box selecting as many or little of the vertices you want. Then you press W to Tolerant Weld all the vertices togeither if there is a gap.

I do this all the time and it makes clearing up large amount of gaps in your terrain so much less painfull.

Sometimes though, it is still best to go in and join some vertices one at a time. Especially very hilly terrain, or odd joins.