In 1905, 250,000 acres in the Cache River watershed were described as wet and worthless for farming. The Cache River Drainage District was created in 1911 with specific purpose of construction the Post Creek Cutoff. By 1916 the 4.8 mile long Cutoff was completed, diverting the 60% of the Upper Cache due south of Belknap into the Ohio River. The fall was to be one foot per mile along the 30-foot wide by 10-foot deep ditch.
The steep gradient and non-meandering route instantly accelerated flows and began a process of severe erosion, which continues today. By 1974, the Cutoff was 200 feet wide and 64 feet deep near the Grand Chain bridge, appearing more like a canyon that a drainage ditch.
Simultaneously, a network of straight ditches and laterals were constructed through the Black Slough region. The Main Brothers Box and Lumber Company used these as float roads to get logs to the sawmill in Karnak. They also diverted part of the Cache River north of Karnak through Sawmill Ditch where logs were stored in a pond.
Following the completion of the Cutoff and miles of additional drainage ditches, thousands of acres of timberland were cut and cleared through the 1940s. Today most of the vast swamp/pond complex exists has huge farm fields, which are tiled with outlets into another ditch, called the Main Ditch and its tributaries.
Post Creek Cutoff causes Environmental Damage to the Eastern Half of the Cache Watershed
The increased water velocities through the Cutoff have resulted in headward gully migration, causing scoured channels and eroded banks 20 miles upstream. This phenomenon directly affected Heron Pond (a National Natural Landmark) when erosion incised the Cache adjacent to Heron Pond by only six feet. With swamp levels higher than the river and bank erosion eliminating the natural levees that once separated the swamps from the river, underground piping and open gullies threaten to drain Heron Pond and the other natural swamps along the Cache.
As the Post Creek Cutoff has deepened and widened, so have the many side streams and ditches that feed into it. The resulting lateral gullies, some over a mile long, extend into adjacent farm fields impeding access and loss of useable farmland.
The Cutoff has lowered the water table and caused the loss of natural springs. The changes in hydrology have caused changes in the composition of natural plant and animal communities. And, because of the loss of the natural flood retention capabilities of the Black Slough, some places downstream experience worse flooding now than in pre-settlement times.
Large silt deposits end up at the mouth of the Cutoff in the Ohio River. Annual dredging is required to keep the navigation channel deep enough for river traffic.
The next article will explore the impacts of the dividing the Cache into two rivers on the Lower Cache in the western half of the watershed.
Source: Resource Plan for the Cache River Watershed, Cache River Watershed Resource Planning Committee, December 1995