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The Geology & Hydrology of the Spanish Lake Basin

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What is a backwater swamp? A backwater swamp is a wetland that acts as a conduit for water. For example, when the Mississippi River overflowed its banks, the Spanish Lake Basin acted as a collecting point for the water (because the area is relatively lower than the surrounding land). This water was dispersed through the bayous and ultimately into Lakes Maurepas and Ponchartrain and the Gulf of Mexico. However, high water from Lake Maurepas could come up the bayous and flood the wetland. Bayous can run both ways. This system connected the Mississippi River to Lake Ponchartrain, a fact that would become very important to the history of southeast Louisiana.

Geologic and hydrologic forces shaped the Spanish Lake Basin, a "bowl" of soil and water sculpted by rivers weathering and eroding the land over tens of thousands of years. Beginning 100,000 years ago, river systems built up a high terrace in the area of the Spanish Lake Basin and carved the valley that later became the Basin. Later, the salty waters of the Gulf of Mexico entered the valley and, as sea levels dropped, it filled with sediment again.

Approximately 4,800 years ago, the Mississippi River changed course, moving eastward from Bayou Teche in the Atchafalaya Basin to a channel running alongside the bluffs of Baton Rouge. From Baton Rouge, the river flowed down its present course past New Orleans into the Gulf of Mexico. Gradually,

the modern river channel built up a natural levee on the western side of this valley. With this development, the valley was surrounded by a near circular ridge and became a distinct geologic formation: a wetland basin, or "bowl," extending from the east bank of the Mississippi River below Baton Rouge, along the high terrace, and back to the river's natural levee. An ecological masterpiece of open lakes, cypress tupelo swamp, bottomland hardwood forests, and distributary ridges was formed.

What is a freshwater catchbasin? A freshwater catchbasin is what the backwater swamp became in the early 1800s when levees were built along the Mississippi River’s banks to help stop the flooding of homes and farms. The Spanish Lake Basin, separated from the river, became a freshwater catchbasin fed only by rainfall and stormwater runoff from the three surrounding parishes.

The Spanish Lake Basin was named for the scenic, mile wide lake near its center. For thousands of years, the Basin was a backwater swamp of the Mississippi River. In recent decades, however, hydrologic modifications (such as draining and filling wetlands and leveeing and channeling waterbodies) and urban development have reshaped and altered the natural processes of the Spanish Lake Basin.

Part Two:
Rising and Falling Waters:

The Spanish Lake Basin is different from many basins in having a flood drain cycle that is governed by three separate hydrologic systems. Together, these systems comprise a huge collecting basin, which receives the stormwater runoff of three Louisiana parishes: East Baton Rouge, Ascension, and lberville.

Extends north from Bayou Manchac (East Baton Rouge Parish). For thousands of years, Bayou Manchac was a distributary and tributary. It carried Mississippi River floodwaters into the Lake Pontchartrain Basin, and when that basin's waters were high, the bayou reversed its flow and emptied into the Spanish Lake Basin! (Hydrologists find this two way movement of bayous quite irregular for a waterway.) However, Bayou Manchac has flowed only one way, into the Lake Pontchartrain Basin, since levees were built along the Mississippi River in 1826. Today, as rain rolls off impervious surfaces such as patios. parking lots.

The western side of the Spanish Lake Basin (Iberville parish) includes the habitats of hardwood forests, cypress tupelo swamps, bayous, and open lakes. This hydrologic system, bordered on the east by a manmade levee, covers most of the Basin and drains into Bayou Manchac.

The Bluff Swamp hydrologic system (Ascension Parish). This system is buffered from the open waters of Cypress Flats by a manmade levee. The swamn's thick vegetation absorbs some stormwater