To secure instream flow at the Yudeung urban stream reach of Daejeon city in South Korea, Yudeung upstream diversion was designed with total water storage of $59{ imes}10^4m^3$, and the upstream Seongol reservoir was planned to raise the bank with various sizes. Downstream streamflows were simulated by considering outflows from upstream diversion and reservoir, and after then flow durations were analyzed and compared with flows of no reservoir condition. In case of no diversion or reservoir upstream, flow durations were $1^{st}$ flow of $84.72m^3/s$, $95^{th}$ flow of $2.10m^3/s$, $185^{th}$ flow of $0.92m^3/s$, $275^{th}$ flow of $0.42m^3/s$, and $355^{th}$ flow of $0.31m^3/s$. In case of upstream diversion, flow durations were $1^{st}$ flow of $94.38m^3/s$, $95^{th}$ flow of $2.96m^3/s$, $185^{th}$ flow of $1.22m^3/s$, $275^{th}$ flow of $0.50m^3/s$, and $355^{th}$ flow of $0.35m^3/s$. The increase flow rates were $0.04m^3/s$ in $355^{th}$ flow, $0.08m^3/s$ in $275^{th}$, and $0.30m^3/s$ in 185th. In case of Seongol reservoir with effective storage capacities of $365{ imes}10^4m^3$, $544{ imes}10^4m^3$, $750{ imes}10^4m^3$, and $992{ imes}10^4m^3$, flow durations were $85.5{sim}83.9m^3/s$ on $1^{st}$ flow, $2.85{sim}2.57m^3/s$ on $95^{th}$ flow, $1.16{sim}1.27m^3/s$ on $185^{th}$ flow, $0.64{sim}0.99m^3/s$ on $275^{th}$ flow, and $0.56{sim}0.94m^3/s$ on $355^{th}$ flow. The increase flow rates were $0.25{sim}0.63m^3/s$ in $355^{th}$ flow, $0.22{sim}0.57m^3/s$ in $275^{th}$, and $0.24{sim}0.35m^3/s$ in $185^{th}$. The more the sizes of upstream reservoirs increased, the $1^{st}$ and $95^{th}$ flows decreased in which coefficients of determination were 0.92, 0.99, respectively and the $185^{th}$, $275^{th}$, and $355^{th}$ flows increased in which coefficients of determination were 0.93 to 0.99.