Introduction and scope
Early in 2005, I was given the task of investigating the historic hydrology of the Cache River Basin. More specifically, the Joint Venture Partnership (JVP) was interested in better understanding the hydrologic regime and, specifically, water elevations in the Cache River prior to alteration by European settlers in the late nineteenth and early twentieth centuries. I have spent considerable effort analyzing the geomorphology of the Cache River Basin because water surface elevations in a wetland, especially a complex riverine system like the Cache’s, are a function not just of water flows, but of the morphology of the wetland as well. Without understanding the processes that formed and now maintain that morphology, we cannot fully understand the interaction between the landforms, hydrology, and biota that make up the Cache River’s ecosystem.
Because the Cache Basin, shown in Figure 1, is large and resources were limited, I focused my attention on the Cache River Valley roughly between Belknap and Ullin. This area contains Eagle Pond, Long Reach, and Buttonland Swamp, and is the subject of interest in management of water surface elevations to protect and preserve the Valley’s environmental values. Geographic names in this part of the Cache Valley are used rather loosely. For this reason, I will use a designation of my own: The Middle Cache Valley (MCV). This section of the Valley, shown in Figure 2, runs from Belknap (and the Post Creek Cutoff) west to the mouth of Big Creek, about two miles (3.2 km) east of Ullin. This section of the Cache is mapped on the Karnak, Cypress, and Dongola USGS 7.5' quads, and, as the crow flies, is about 10 miles (16 km) long.
As I approached the end of my work on this study and struggled to explain the unusual character of the Cache Valley, I wondered whether shallow subsidence had perhaps formed the MCV’s extensive presettlement wetlands. My inquiries led to seismologists, including Martitia Tuttle and W. John Nelson, who confirmed that there appears to be a good possibility, as yet unexplored, that the MCV is a “sunkland,” i.e. a swamp formed by subsidence or faulting during an earthquake. Sunklands are common in lowland river systems south of this region that were strongly influenced by the New Madrid earthquakes of 1811 and 1812. Aside from the MCV’s unusual long profile, in which the MCV appears vertically depressed and out of equilibrium with the rest of the Cache River’s long profile, there appear to be numerous circular craters in the Cache’s floodplain, including many concentrated along the Cache River just south and east of Perks in sections 11 and 12. These features may be sand blows that form in alluvial plains during strong earthquakes.
I was unable to determine whether the MCV has been influenced by seismic events, but we hope ongoing investigation will answer that question. Though this answer would be important in understanding the Cache River’s natural history, it does not influence the conclusions in this report with regard to presettlement hydrology
