Large wood loads in channels and on floodplains after a 500-year flood using UAV imagery in Mark Twain National Forest, Ozark Highlands, Missouri

Abstract

Anthropogenic climate change has increased the frequency of large floods in rivers draining the Ozark Highlands. This study assesses the effects of a > 500-yr flood in spring 2017 on riparian forests and large wood loads in the North Fork of the White River watershed, Missouri, for six stream reaches with drainage areas from 5 to 124 km2. Standing trees and large wood (LW) were assessed using unmanned aerial vehicle (UAV) imagery and calibrated by field surveys. Scaled flood magnitude (flood stage/bankfull depth) correlated with percent urban and agricultural land above each reach suggesting that land use may have contributed to forest damage. Canopy loss on the valley floor ranged from 7 to 63 % by reach and correlated with mean and cross-sectional stream power (p < 0.01). Standing tree density after the flood ranged from 50 to 243 trees/ha. The density of LW pieces ranged from 25 to 147 trees/ha. Most LW was aligned with stream flow, not in jams, and located on floodplains below riffles or bar heads, along channel bends, or in chutes. Wood loads on the valley floor increased downstream from 12 to 45 m3/ha. Channel loads were < 30 m3/ha while floodplain, terrace, and chute loads were > 30 m3/ha at drainage areas >50 km2. Channel LW loads increased with flood magnitude and in narrow valleys (p < 0.02), but not drainage area. Increased wood storage occurred on floodplains and terraces, but it is not clear if the stored wood will be available for downstream transport by future floods.