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In areas where forests are inundated with seasonal rains, as in parts of the Amazon basin, or located in tidal or mangrove areas, water can be used to extract logs from felling sites. Normally, cutting is done during the dry season or at low tide, and the logs are floated out as the water rises. Loss of logs can be high with such systems, so careful control is needed to avoid wasting a large part of the resource. For dense timbers, felling must be done well in advance so that the logs have enough time to dry out and float better as a result. In tidal areas, the trees may need to be killed in advance of the operation, usually by girdling, and then left to dry out for a period before cutting and extraction. Especially dense logs may need to be tied securely on to rafts of more buoyant logs to prevent them from sinking. Environmental impacts with water-extraction systems include those associated with direct felling damage as well as eventual accumulation of bark, scattered residues on the river banks or woody debris in the waterways. Where tidal flushing occurs or streams are flowing quickly, this accumulation should not pose problems. The decay of bark and woody debris removes free oxygen from the water, however, and this can adversely affect aquatic life, especially in slow-moving streams. Perhaps more significant are environmental problems associated with the extensive cutting of noncommercial trees for use as floaters in rafts. One estimate is that for each commercial tree extracted, another tree was also felled. Soil damage along stream banks also occurs frequently when the logs slide or are rolled into the stream.