erc-input-line-position

erc-input-line-position is a variable defined in `erc-goodies.el'.
Its value is

nil

Documentation:
Specify where to position the input line when using `erc-scroll-to-bottom'.

This should be an integer specifying the line of the buffer on which
the input line should stay. A value of "-1" would keep the input
line positioned on the last line in the buffer. This is passed as an
argument to `recenter'.

You can customize this variable.