We reviewed papers dealing with the effects of land drainage on streamflow, conclusions on effects are difficult to establish because response varies with different types of drainage work and differing physiographic and climate situations. Coincident changes in land use, and the high variability of inputs, add complications.For the range of events giving rise to most floods a combination of subsurface drainage and outlet enlargement has little effect on flood peaks. Flood peaks are reduced when high-water table areas are drained. Connection of swamps or enclosed areas to rivers by large arterial drains increases flood peaks.
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