Headwaters of Bayou Lafourche EarthCache

3000 years ago, the Mississippi River flowed eastward past this location through modern-day New Orleans to the St. Bernard Delta. To the south of this location was a large bay between the remnants of the abandoned Teche Delta and a lobe of the active St. Bernard Delta. (See figure.) As the St. Bernard Delta grew longer, the river became less steep, which caused it to slow. Consequently, the silting of the delta accelerated, which slowed the river further. Around 2500 years ago, conditions became favorable for a drastic change in the river's course, a process known as avulsion [1]. Thus Bayou Lafourche started its existence 2500 years ago as the new course of the Mississippi River. The resulting Lafourche Delta filled in the bay and created most of the land in Lafourche and Terrebonne Parishes as shown in the figure. Around 800 years ago, the course of the Mississippi River shifted eastward once more and the Plaquemines Delta was born, while the Bayou Lafourche watercourse was not completely abandoned. [2, 3]

Evolution of the southeastern Louisiana coastline. (Adapted from [3].) In 1967, D. E. Frazier studied hundreds of sediment cores from southeastern Louisiana. He observed the fine-grained prodelta layers, sandy delta-front layers, and fine-grained delta-plain layers that include peat accumulations from marshes. He used rediocarbon dating of this last layer to establish the chronology of Mississippi River deltas shown in this figure. [4]

In the 18th century, many Acadians and Germans migrated into the Lafourche Valley, which at that time carried an estimated 50,000 cubic feet per second (12% of the flow of the Mississippi River). The Acadians called the smaller branch of the Mississippi "the fork" (Fr. la fourche). These early settlers recognized the same advantages of living along the bayou as the Native Americans that had inhabited the region for centuries before: moderate flood protection and rich soil. The new settlers also utilized Bayou Lafourche as a source of fresh water and a route to the trading center of New Orleans. By the turn of the 20th century, however, the railroad connected the bayou residents to New Orleans. The bayou was no longer critical to transportation and frustration with spring flooding was growing.

In February 1904 a dam was completed on the banks of the Mississippi River at Donaldsonville, severing Bayou Lafourche from the river. The residents of Bayou Lafourche had initially requested the installation of a set of locks at Donaldsonville, but the locks were never constructed. The Lafourche Delta was cut off from the supply of sediment that sustained it. Without a supply of sediment the land in the delta began to sink as the underlying sedimentary layers compacted. The lack of fresh water from the Mississippi River was exacting its toll on the residents as well. Rainfall in the Bayou Lafourche watershed was insufficient to provide bayou residents with drinking water. To exacerbate the drinking water problem, the flow in Bayou Lafourche was inadequate to prevent the encroachment of salt water from the Gulf into the bayou. [5]

Walter Le Mann, a citizen of Donaldsonville, led the efforts to restore a flow of fresh water to Bayou Lafourche, which led to the construction in 1955 of the pumping station that you see before you. However, at 300 cubic feet per second, the pump provides only a fraction of what once flowed down Bayou Lafourche from the Mississippi River. Currently, plans are being studied to modify the channel of the bayou to accommodate a larger flow [7]. Another problem is that the pumping station does not provide sediment to offset land subsidence. Another plan – referred to the Third Delta Conveyance Channel – is being studied to convey sediment into the marshes to offset subsidence. [8 ]

To claim this Earthcache:

1. Roughly estimate the width of Bayou Lafourche. E-mail the answer to me.
2. Using an estimated average depth of 5 ft., calculate the flow speed in the bayou using the equation
   300 ft³/s = (width in feet) X (average depth in feet) X (flow speed in feet/second).
   Convert to miles/hour. (1.5 ft/s = 1 mile/hour). E-mail the answer to me.
3. Roughly estimate the height to which the banks of Bayou Lafourche rise above the current water level. E-mail the answer to me.
4. (For those who want an extra challenge. Not necessary for claiming the cache.)
   For a wide, rectangular channel (the simplest case), the depth and flow rate are related by the following equation [9]:
   (new depth)/(old depth) = ((new flow rate)/(old flow rate))^2/3.
   If the flow rate is increased to 1000 cubic feet per second, what do you expect the new depth of Bayou Lafourche to be? (Barring the effects of bridges, culverts, dredging, trees, etc.) E-mail the answer to me.
5. (Optional) Take a picture with a distinctive feature of the site in evidence. Post this with your log.

If you leave the site by way of Iberville St., you can get a closer view of the headwaters of Bayou Lafourche. Please do not approach the headwaters any closer than Iberville St. The Bayou Lafourche Freshwater District has specifically prohibited access to this area for safety reasons. You can get an unobstructed view of Bayou Lafourche by turning right from Iberville St. onto Lafourche St. and following Lafourche St. for a short distance until it becomes La. 308.

Sources:

Acknowledgement: Permission to place this Earthcache was granted by the Bayou Lafourche Freshwater District.