A trapezoidal channel with bottom width $b=8$ and side slopes of $z=3H$:$1V$ and a slope of $0\mathrm{.}0064$ is carrying a flow of $160\mathrm{.}6cfs.$ The Mannings $n$ value is $0\mathrm{.}040.$ For one step of the backwater calculations the known downstream depth is $1\mathrm{.}91.$ The calculated depth $3\mathrm{.}0ft$ upstream is

$($answer in feet to $2$ decimal places$).$