HISTORICAL FLOOD VARIABILITY AND GEOMORPHIC EFFECTIVENESS IN OZARK PLATEAU RIVERS

It is widely recognized that flood regime can influence the geomorphic behavior of river channels. Flood magnitude and frequency relationships are typically used to explain channel adjustments caused by land use disturbance and/or climate change in watersheds. However, less attention has been given to climate-driven changes in flood regime over the past 50 years and their potential influence on contemporary channel morphology. This study evaluates the historical and recent variability of flood magnitude and frequency at 19 long-term discharge monitoring stations on rivers within the Ozark Plateaus region of southern Missouri. Flood frequency distributions for the annual maximum series were calculated over 30 year periods at 5 year intervals from 1922 to 2012 to examine temporal trends of flood magnitudes ranging from the 1 to 100 year recurrence intervals. Gage records typically exhibited a >20% increase in bankfull discharge since 1950, even in forested watersheds. The 100-year flood record varied among gages to some degree, but most indicated an increase in large floods over the past 20 years. Implications of these flood trends on geomorphic processes and channel change in the Ozarks are discussed. For example, ecological and water quality problems caused by excessive gravel bar sedimentation and related channel instability in the Ozarks may be explained by climate-change influence and may not need land use or internal threshold drivers.