Transport and mixing of momentum and salt in an estuary varies in time and space due to river discharge, changes in tidal amplitude and phase, wind stress, and lateral mixing processes, such as secondary circulation. This dissertation focuses on observing, describing, parameterizing, and quantifying secondary circulation in a sinuous coastal plain estuary using acoustic Doppler current meters. This endeavor is made to improve our general understanding of secondary circulation, to identify its primary driving forces, and to better parameterize key physical processes necessary for further study in current numerical models. Secondary circulation is used to describe several mechanisms whose result is to vertically overturn the water column along the transverse axis of a channel. Secondary circulation is commonly generated by one or more of the following mechanisms: channel curvature, unusual bottom topography or channel geometry, planetary rotation, and/or the differential advection of density. Data for this dissertation was collected in the naturally sinuous Satilla River in southeast Georgia. A shallow coastal plain estuary, the Satilla is a partially-mixed estuary characterized by 2 meter range semidiurnal tides. It has a strong neap-to-spring axial current inequality and strong neap-to-spring vertical salinity differences. The balance of mechanisms responsible for the strength and location of secondary flow in the Satilla River varies with spatial location, phase of the tide, and lateral cross-channel position. A simple steady-state momentum balance between the Coriolis acceleration, centrifugal acceleration, the lateral baroclinic gradient, and bottom stress is sufficient to explain secondary circulation in the Satilla River under a wide variety of conditions. The primary momentum balance for this river is a three-way balance between the centrifugal acceleration, bottom stress, and the lateral baroclinic gradient. The dominant mechanism that drives the local secondary circulation depends on the phase of the tide and the lateral placement of the acoustic current profiler.