From "The Brewers Dictionary":
Channelling- Drawing off water at a faster rate than replenished through the sparging process, will cause compacting of the grain bed which results in channeling, and a diluted wort. Channeling is caused by water flowing around the grain rather than through it. When grain becomes compacted, the water tends to flow the path with the least resistance, which can mean a ruined batch of beer or an inadvertent "small beer" that has a lower original gravity than the brewer intended.