The hydrography of the Broadkill River, a narrow, shallow estuary in southeastern Delaware, was studied from July, 1966, through February, 1971. Measurements of current velocity, tide stage, salinity, water temperature, dissolved oxygen, pH, and turbidity were made at several stations along the course of the estuary. The Broadkill River has characteristics of both a partially-mixed and a well-mixed estuary. During normal runoff the system discharges 1.3×105 m3 of freshwater seaward per tidal cycle. The flushing time of the entire system increases from 10.3 tidal cycles during high runoff to 24.2 tidal cycles during low runoff. For purposes of analysis, the Broadkill River was divided into three sections: the lower and upper estuaries, and the tidal river. The lower estuary, the most seaward 25% of the system, is sectionally homogeneous during 90% of each tidal cycle. Homogeneity is maintained by strong tidal currents. Vertical salinity and water temperature stratification occurs only during a short interval of simultaneous two-directional current flow at the beginning of each flooding period. Salinity, water temperature, dissolved oxygen, pH, and turbidity generally reflect conditions in the adjoining lower Delaware Bay. Net non-tidal current flow is seaward at all depths. The flushing time of the lower estuary is relatively constant and increases from 0.6 to 0.9 tidal cycle with decreasing freshwater runoff. The upper estuary, the central 35% of the system, has the most distinct longitudinal and vertical salinity gradients. Tidal currents are slower than those in the lower estuary. Net non-tidal flow may be either two-directional or seaward at all depths. Dissolved oxygen and pH generally decrease and turbidity increases with distance upstream through this section. The flushing time of the upper estuary increases from 3.2 to 4.6 tidal cycles with decreasing freshwater runoff. The tidal river, the farthest upstream 40% of the system, is rarely invaded by saline water from the upper estuary. This section is polluted through the discharge of organic wastes into the river at a point 0.5 km downstream from the river’s source, Wagamon’s Pond at Milton, Delaware. Downstream from the source of pollution, anoxic conditions are present from spring through autumn and are relieved only during the coldest months of the winter. The lowest pH readings and greatest turbidities also occur in this section of the tidal river. The discharge from Wagamon’s Pond dominates the tidal river upstream from the source of pollution. In this section dissolved oxygen rarely decreases below 8 mg/liter. The pH readings are generally higher than those in the anoxic section, and the water in this section is the least turbid of any water in the Broadkill River. The slowest tidal currents occur in the tidal river, and net non-tidal flow is generally seaward at all depths. The flushing time of the tidal river increases from 6.7 to 18.7 tidal cycles with decreasing freshwater runoff.