"Channel" has many meanings. The one she is using here is that of the bed for a waterway. A channel can be large or small. For example, the English Channel is a very large channel that runs between England and continental Europe. However, channels can be very small as well. A tiny brook or stream runs through a channel. That is the literal meaning upon which her mataphoric meaning is built. Water is often used as a metaphor for life. Poets sometimes compare our lives to the crossing of a sea. Our lives flow like water through rivers and streams. To Helen Keller, the books she has read are like channels that have allowed her life to flow and given her meaning.