Role of wood debris in modifying channel geomorphology and riparian areas of a large lowland river under pristine conditions: A historical case study

Abstract

Summary Historical records were reviewed to estimate the impact of wood debris on a 400–500 km reach of the Red River, Louisiana, U. S. A. In the primal condition the banks consisted of unstable alluvial deposits easily eroded during floods. Sediment and riparian trees from eroded banks formed organic debris dams which blocked the channel and promoted channel aggradation. Over a period > 375 years wood debris dams formed in series. The longest instantaneous length of impacted channel was 225 km. The average rate of channel blockage was 1.3–1.6 km · year⁻¹ between 1793–1876. Maximum debris accumulation recorded in a single flood was 8.1 km. Exposed wood covered 80– 120 km of channel. Debris dams remained in place 80–150 years. Wood Debris Impacted Adjacent Riparian Areas by Flooding Forests and Forming a Series of Large Lakes. Under Natural Conditions Lakes Could Become Nearly Permanent Features of the Riparian Landscape If Tributary Channels Filled with Alluvium and Organic Debris. After Removal of Organic Debris the Most Recently Formed Lakes Drained Over a Period of Approximately 30 Years. Impoundment of Water in the Main River Channel Reversed the Direction of Flow in Many Tributary Streams, Especially at High Discharge. Flow Reversal in Tributaries Resulted in Channel Enlargement and Diversion of One-Half to Three-Quarters of the River's Discharge Adjacent to Riparian Lowlands. Wood Debris Reduced the River's Width from Approximately 185 M to Approximately 40 M, and Aggraded the River Bed a Maximum of 7 M. Tributary Channels Were Dammed or Filled with Organic Debris, and the River Was Permanently Opened to Navigation in 1873. Restoration of Full Channel Flow Exposed Previously Buried Logs and Eroded Forested Banks. By 1904, seventy years of debris dams and snag removal, levee projects, dredging and cutting bankside trees resulted in a cleared, wide, meandering channel, which might today be mistaken as typical of a pristine lowland river.