A rotating drum screen channel installation, comprising: a channel in which water flows, a drum for screening out debris from the flow of water in the channel, said drum rotating on a horizontal axis, with a shaft supporting the drum for rotation, the shaft being supported on fixed structure of the channel, and a screen extending around the drum's circumference, forming the drum screen, the axis of the drum screen being arranged perpendicular to the channel flow direction characterized in that the drum has a width that occupies part of a width of the channel, leaving a remaining width of the channel for water flow the drum screen having a diameter not greater than about 3 m (10 feet), and being a single entry drum screen, with an open side facing said remaining width of the channel and positioned to receive a flow of water from said remaining width of the channel, the channel having a flow direction from upstream to downstream, and said remaining width of the channel being blocked by a barrier adjacent to a downstream end of the drum screen, said barrier being adapted to close the channel between a drum seal plate of the drum screen and a wall of the channel, wherein the drum seal plate is positioned in the channel alongside the open side of the drum screen, with a flexible contact seal between the seal plate and a circular side edge of the rotatable drum screen, the seal plate having a water admitting opening to permit said flow of water into the drum screen from the open side, a diverter plate in the channel, with a downstream end of the diverter plate connected to an upstream end of the seal plate, the diverter plate having an upstream end fixed to a wall of the channel and engaged against a floor of the channel, a main portion of the diverter plate being angled so as to form a parallel flow offset to direct all water and debris flowing in the channel into said remaining width channel prior to entering the drum screen through said water admitting opening, and the drum screen installation including a debris hopper within the drum, fixed in position near a highest point of the drum, for catching debris from the screen as the drum screen rotates, and with a series of high-pressure water nozzles above the drum aimed at the hopper, to dislodge the debris from the screen into the hopper, and further including an exit trough positioned to carry away the debris and water from the hopper. a channel in which water flows, a drum for screening out debris from the flow of water in the channel, said drum rotating on a horizontal axis, with a shaft supporting the drum for rotation, the shaft being supported on fixed structure of the channel, and a screen extending around the drum's circumference, forming the drum screen, the axis of the drum screen being arranged perpendicular to the channel flow direction characterized in that the drum has a width that occupies part of a width of the channel, leaving a remaining width of the channel for water flow the drum screen having a diameter not greater than about 3 m (10 feet), and being a single entry drum screen, with an open side facing said remaining width of the channel and positioned to receive a flow of water from said remaining width of the channel, the channel having a flow direction from upstream to downstream, and said remaining width of the channel being blocked by a barrier adjacent to a downstream end of the drum screen, said barrier being adapted to close the channel between a drum seal plate of the drum screen and a wall of the channel, wherein the drum seal plate is positioned in the channel alongside the open side of the drum screen, with a flexible contact seal between the seal plate and a circular side edge of the rotatable drum screen, the seal plate having a water admitting opening to permit said flow of water into the drum screen from the open side, a diverter plate in the channel, with a downstream end of the diverter plate connected to an upstream end of the seal plate, the diverter plate having an upstream end fixed to a wall of the channel and engaged against a floor of the channel, a main portion of the diverter plate being angled so as to form a parallel flow offset to direct all water and debris flowing in the channel into said remaining width channel prior to entering the drum screen through said water admitting opening, and the drum screen installation including a debris hopper within the drum, fixed in position near a highest point of the drum, for catching debris from the screen as the drum screen rotates, and with a series of high-pressure water nozzles above the drum aimed at the hopper, to dislodge the debris from the screen into the hopper, and further including an exit trough positioned to carry away the debris and water from the hopper.