LOWER ALTAMAHA WATERSHED

        The entire Altamaha Basin is the largest watershed in the State of Georgia draining about 1/4 of the state area. It is also the third largest basin draining to the Atlantic Ocean. It is formed by the confluence of Ocmulgee, Oconee and Ohoopee rivers. From the confluence point of Ocmulgee and Oconee rivers down to the Atlantic Ocean, the river system is known as the Altamaha river. It is only this most downstream part of this large basin, which is referred as the Lower Altamaha watershed. This study focuses on this section of the entire Altamaha basin. During 02-03 June 2001, a field trip is conducted to obtain a better understanding of the Lower Altamaha watershed. The highlights of this trip are presented in our Field Trip page. Please check it out for visual aids on the Lower Altamaha watershed.

       Administratively, the Lower Altamaha watershed is divided by 10 counties: Jeff Davis, Montgomery, Toombs, Appling, Tattnall, Evans, Long, Wayne, McIntosh and Glynn. A comparative table of the Lower Altamaha watershed, the 10 counties this watershed resides and the state of Georgia is given in Table 1. It is seen from Table 1 that even though the watershed covers about 2.5% of the state's total area, only 0.7% of the state's total population reside in the watershed, which gives a low population density.

Table 1. Surface Area and Population Characteristics of Lower Altamaha

        The lower Altamaha has an drainage area of about 1,500 square miles compared to the total of 13,600 square miles of the entire Altamaha. Looking at the overall hypsographic features of the entire Altamaha basin, one can see that, with an average width of 16 miles, the Lower Altamaha basin is like an 'outflow pipe' or a 'conduit'  discharging the combined flows of Ocmulgee and Oconee. The basin has an average elevation of 165 ft and an average annual precipitation of approximately 45 inch. At the USGS Gaging Station near Doctortown, GA, the long term mean annual flow is measured to be around 14,000 cfs.

        The lower Altamaha watershed shows all characteristics of a typical lowland with gentle slopes. The maximum elevation difference between the lowest and highest points of the watershed is approximately 300 ft representing a very mild topography. The main feature of the drainage pattern shows a meandering pattern in this low land area. Wetlands are commonly observed in this low gradient areas, especially along the river banks and in the coastal region. A contour map showing the Lower Altamaha watershed illustrates three distinct topographic zones: (i) the (relatively) highlands; (ii) the transition zone, and (iii) the lowlands or the coastal plains. This map also shows the gradual increase in floodplain width as river flows towards the lowland zone.

        The Lower Altamaha River is the main drainage feature in the basin. It has an average slope of 0.00018 ft/ft and a gently meandering nature. This meandering pattern of the Altamaha River can be seen from the figure to your left. The drainage pattern given in this figure is created from an EPA Reach File Version 3 (RF3). Especially in the coastal plains, the drainage pattern becomes extremely complex to analyze. The Lower Altamaha River does not have a significant drainage area but rather acts like a 'conduit' to convey the combined flows of Ocmulgee, Oconee and Ohoopee. The average watershed width of 16 mi supports this finding.

         The soil data for the watershed is directly used in the infiltration computations that are used to compute the excess rainfall. The Lower Altamaha watershed is characterized by (i) loam; (ii) sand, and (iii) loamy sand type soils. The loamy sand soils are mostly seen in the highlands, where as the sand soils are commonly observed in the transition zone. It is also possible to see that the organic matter and loam type soils are the most common soil type along the Altamaha River floodplain. The soil map given to your right is created from the US Department of Agriculture Natural Resources Conservation Service's State Soils Geographic Database (STATSGO). 

        The land cover/use of the Lower Altamaha watershed is another important source of information for hydrologic models. The overland flow resistance is a direct function of the type and density of the vegetation present on the land and the type of land use practices applied. Focusing on the land cover/use map given to your left, it can be seen that woody wetlands dominate the floodplain area of the Altamaha River (see some of the pictures taken during the field trip). Typical forest cover of the watershed is another common land cover seen in the field trip pictures.

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