Ecological dynamics of wetlands at Lisbon Bottom, Big Muddy National Fish and Wildlife Refuge, Missouri

Abstract

Lisbon Bottom consists of approximately 875 ha of river bottom along the Missouri River in Howard County, Missouri, from approximately river mile (RM) 213 to RM 219 (figs. 1-1–1-3). Before regulation and structuring of the Missouri River, riverine areas like Lisbon Bottom were shifting mosaics of dynamic habitat patches that were created and maintained by hydrologic and geomorphic processes. Flow regulation, navigation structures, and bank-stabilization projects isolated Lisbon Bottom from the river by decreasing the magnitude and frequency with which hydrologic and geomorphic processes could alter habitat characteristics. The flood of 1993 breached agricultural levees around Lisbon Bottom, creating numerous levee-break scours, and reestablishing a connection to the Missouri River. Management of wetland resources requires an understanding of how water recharges wetlands, how quality of the water may vary with source, and the costs and ecological benefits associated with manipulations of water sources. Observations and monitoring of surfaceand ground water at Lisbon Bottom indicate the relative contributions to wetlands of water from the main channel by overbank flooding, water from the main channel by subsurface connection, water from direct rainfall, and water discharging from valley-wall tributaries. These sources of water are distributed among the many types of wetlands that exist at Lisbon Bottom, including deep scours formed during the 1993 and 1995 floods, shallow temporary wetlands with minimal direct surface drainage area, and shallow temporary wetlands with direct surface-water connections to valley-wall tributaries. Deep scours associated with levee breaks and crevasse splays at upstream and downstream ends of Lisbon Bottom are connected through subsurface flow to the main channel. Water levels in these scours vary with flow in the main channel, direct rainfall, and to a lesser extent, valley-wall tributaries, but because of ground-water sources, the water levels change slowly over time. Because the down-valley gradient of Lisbon Bottom is greater than the channel slope, flooding in the main channel recharges downstream scours through surface-water flow before recharging upstream scours. Wetlands along the valley wall, far from the main channel, are recharged by main-channel flow only when flow is well over bank. These wetlands are recharged more frequently by local rainfall that falls directly into surficial drainage areas and by flow from valley-wall tributaries. Hydrologic variation in wetlands that are recharged by local rainfall is of greater magnitude and much more frequent than variation in deep scours, resulting in markedly different hydrologic disturbance