Disturbance and recovery of large floodplain rivers

Abstract

/ Disturbance in a river-floodplain system is defined as an unpredictable event that disrupts structure or function at the ecosystem, community, or population level. Disturbance can result in species replacements or losses, or shifts of ecosystems from one persistent condition to another. A disturbance can be a discrete event or a graded change in a controlling factor that eventually exceeds a critical threshold.The annual flood is the major driving variable that facilitates lateral exchanges of nutrients, organic matter, and organisms. The annual flood is not normally considered a disturbance unless its timing or magnitude is "atypical." The record flood of 1973 had little effect on the biota at a longterm study site on the Mississippi River, but the absence of a flood during the 1976-1977 Midwestern drought caused short-and long-term changes. Body burdens of contaminants increased temporarily in key species, because of increased concentration resulting from reduced dilution. Reduced runoff and sediment input improved light penetration and increased the depth at which aquatic macrophytes could grow. Developing plant beds exerted a high degree of biotic control and were able to persist, despite the resumption of normal floods and turbidity in subsequent years.In contrast to the discrete event that disturbed the Mississippi River, a major confluent, the Illinois River, has been degraded by a gradual increase in sediment input and sediment resuspension. From 1958 to 1961 formerly productive backwaters and lakes along a 320-km reach of the Illinois River changed from clear, vegetated areas to turbid, barren basins. The change to a system largely controlled by abiotic factors was rapid and the degraded condition persists.Traditional approaches to experimental design are poorly suited for detecting control mechanisms and for determining the critical thresholds in large river-floodplains. Large riverfloodplain systems cannot be manipulated or sampled as easily as small streams, and greater use should be made of man-made or natural disturbances and environmental restoration as opportunistic experiments to measure thresholds and monitor the recovery process.Rivers with floodplains differ from rivers and streams with constrained channels and narrow riparian zones. Differences include flooding patterns, relative importance of lateral versus upstream-downstream linkages in nutrient cycling (or spiraling), fluxes of organic matter, and the adaptation of many biota to transient aquatic-terrestrial environments (Junk and others 1989). These differences govern how we understand disturbance and how we design monitoring programs, manage and regulate water quality and quantity, and restore degraded riverfloodplain systems.The fact that all large rivers in the temperate zone

Restoring ecological integrity of great rivers: historical hydrographs aid in defining reference conditions for the Missouri River

“…There was historically a small fall increase in discharge along the lowermost Missouri River, similar to that reported by Sparks et al (1990) and Sparks (1995) for the nearby upper Mississippi River. This small pulse is regional and due to the onset of autumn rains in the well-watered Central Lowlands and Interior Highlands physiographic provinces (Galat et al, 1998).…”

Early Cretaceous vegetation and climate change at high latitude: Palynological evidence from Isachsen Formation, Arctic Canada

“…We infer the vertical (y-axis) of the biplot to represent disturbance based on the botanical affinities of taxa and their position on the plot. A disturbance, such as periodic flooding, can be discrete events or a change in a factor influencing an ecosystem that eventually exceeds a critical threshold and results in a shift from one persistent condition to another (Sparks et al, 1990). Moisture availability and disturbance are main drivers of vegetation dynamics in modern coastal lowland settings (Bledsoe and Shear, 2000) and were also fundamental influences ecosystems during the Mesozoic (Abbink et al, 2004;Stukins et al, 2013).…”

Identifying and quantifying environmental thresholds for ecological shifts in a large semi-regulated river

“…Once particular thresholds are crossed, aquatic systems can move away from desired ecological conditions and it can become very difficult to shift the system back to the desired state (Groffman et al 2006). Managers need to know where these thresholds exist due to the very high stakes associated with crossing the ecological tipping points (Sparks et al 1990;Scheffer & Carpenter 2003). I identified thresholds ranging between 12.29 and 19.26 mg/L mean summer TSS for the UMR.…”