Flood is essential for the regeneration and growth of floodplain forests in arid and semiarid regions. However, river flow, especially the flood, had changed greatly with the increase of water diversion from rivers and/or reservoir regulation, which may result in severer deterioration of the floodplain ecosystems. The estimation of appropriate flood stage to inundate the floodplain forests is necessary for their restoration or protection in flow regulation. To balance the economical water use and flood for the floodplain forest, we proposed the inundated forest width method to estimate minimum flood stage for floodplain forests from the inundated forest width – stage curve. The minimum flood stage was defined as the breakpoint of the inundated forest width – stage curve, and could be determined directly from the curve or analytically. For the latter case, the problem under consideration was described by a multi-objective optimization model, which can be solved by the ideal point method. In order to protect the forest in a river floodplain in semiarid area in Xinjiang that subjected to reservoir regulation in the upstream, the proposed method was used to determine the minimum flood stage and flow for the forest. Field survey on hydrology, topography and forest distribution was carried out at representative cross sections in the floodplain. Based on the surveying results, minimum flood flows for six representative cross sections were estimated to be 306 m3/s to 393 m3/s. Their maximum, 393 m3/s, was taken as the minimum flood flow for the studying river reach. It provides an appropriate flood flow for the protection of floodplain forest and can be used in the regulation of the upstream reservoir.
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