A trapezoidal channel with bottom width $b=10$ and side slopes of $z=0H$:$1V$ and a slope of $0\mathrm{.}0049$ is carrying a flow of $89\mathrm{.}2$ cfs$.$ The Mannings $n$ value is $0\mathrm{.}035.$ For one step of the backwater calculations the known downstream depth is $2\mathrm{.}68.$ The calculated depth $40\mathrm{.}0ft$ upstream is $($answer in feet to $2$ decimal places$).$

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