Seasonality and interannual variability of freshwater inflow to a large oligohaline estuary in the Northern Gulf of Mexico

Abstract

Quantity, timing, duration, and fluctuation of freshwater inflow are important factors affecting the development and health of aquatic and adjacent wetland ecosystems in coastal estuaries. This study assessed six decades of freshwater inflow from the Amite River, Tickfaw River, and Tangipahoa River watersheds to Lake Pontchartrain, a large oligohaline estuary in the Northern Gulf of Mexico, whose flood waters caused recent damage to the city of New Orleans in the aftermath of Hurricane Katrina. By utilizing the long-term (1940–2002) river discharge and climatic data from the three major tributary watersheds, monthly and annual freshwater inflows have been quantified and their spatial and temporal variations have been analyzed. On average, the three rivers discharged (±standard error) 0.27 ± 0.04 km 3 freshwater monthly and 3.29 ± 0.15 km 3 freshwater annually into the lake estuarine system, with the highest inflow from the Amite River (0.16 ± 0.03 m 3 mon −1, and 1.91 ± 0.09 km 3 yr −1) and the lowest inflow from the Tickfaw River (0.03 ± 0.00 km 3 mon −1, and 0.34 ± 0.02 km 3 yr −1). A distinct seasonality was evident with over 69% of the total annual inflow occurring during December and May (wet months) and with a low flow period from August to November (dry months). The monthly inflow during the wet months was positively correlated with the monthly precipitation ( r 2 = 0.64), while the monthly inflow during the dry months was subject to evapotranspiration. Furthermore, the study found a 20-year low flow period from 1954–1973 (2.76 ± 0.24 km 3 yr −1) and a 24-year high flow period from 1975–1998 (3.84 ± 0.24 km 3 yr −1), coinciding with both the climate variation and population growth in the watersheds.