Chevrolet 2003 Avalanche Owner's Manual page 148

Side Rail Channels
The side rail channels are located on top of both sides
of the cargo area. You may want to flush them out
with clean water if you notice any debris collecting inside
of them.
When loading cargo into the cargo area, be careful not
to damage the rails.
Midgate Drain Grate Removal and
Cleaning
The midgate drain grate is located near the base of the
midgate (the midgate side facing outside). You will
find a removable drain grate covering the drain.
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