Influences of channel and floodplain modification on expansion of woody vegetation into Catahoula Lake, Louisiana, USA

Abstract

Ecosystem structure of wetlands in managed floodplains depends on hydrological processes controlled by geomorphology and water management. Overlapping effects of direct modifications and geomorphic adjustments to management can combine to trigger changes to floodplain ecosystem structure. We examined the case of woody vegetation encroaching into the depressional Catahoula Lake, Louisiana, in the context of regional hydrologic and geomorphic modification in the floodplain of the Mississippi River. Historical aerial photographs indicated woody encroachment into Catahoula Lake for at least 80 years, and the rate of expansion has increased in recent decades. Historical stage analysis revealed that the downstream Red–Atchafalaya–Mississippi River system controls the lower limit of the lake water level when the large rivers are high, but channel enlargement and other hydrological changes there have reduced the frequency of backwater flooding by 42% since 1880. In addition, operation of the water control structure on the lake has altered its hydrological regime to be more regular among years. Historic stage analysis revealed current lake levels are lower in the high‐water spring, less variable in the dry period, and lack the extreme high‐water events of 100+ years ago, all of which facilitate the expansion of woody vegetation.

Lowland rivers: Geomorphology, human impacts, and management

“…Because of a long period of hydrologic mismanagement lower quality habitat comprised of woody vegetation has encroached over herbaceous wetland flora, which supports much higher levels of biodiversity and biological functions. Woody vegetation encroachment is occurring at increased rates, as revealed with historic aerial photography over the past eight decades (Keim et al, 2022). Since 1880 backwater flooding of the basin has reduced by some 42%.…”

“…Because of their vast size, fine‐grained deposits, and low relief, flood basins are associated with complex inundation processes. This includes overbank, groundwater, local (small) tributaries, pluvial sources, and at the coast, sea level rise (Hudson, 2021, p. 48; Keim et al, 2022; Phillips, 2022). And flood basin inundation can be exacerbated by ground subsidence, particularly when soft organic‐rich deposits undergo oxidation due to embankment and drainage.…”

“…The 10 special issue contributions herein reviewed concern important fundamental and applied topics within lowland river science, specifically highlighting three key themes. These include ‘natural’ river channel dynamics within settings that vary from the standard body of knowledge (Guha et al, 2022; Rogério et al, 2022; Sinha et al, 2022), geomorphic response to channel engineering (Mossa & Chen, 2022; Marteau et al, 2022; Tang et al, 2022; Wu et al, 2022), and flood basin management in response to environmental change (Keim et al, 2022; Phillips, 2022; Valentine et al, 2022). The categories are not mutually exclusive, and most contributions have linkages across multiple themes.…”