dead summer time

Cub Pilot

Senior Member
Hi there,
please, what's meant by dead summer time in this sentence (Mark Twain, 'Life On The Mississippi', Frescoes From The Past)?

They have found a fragment of a lumber raft (it is high water and dead summer time), and are floating down the river by night, and hiding in the willows by day, - bound for Cairo,- whence the negro will seek freedom in the heart of the free States.

I much appreciate your help. Thanks in advance:)
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