estuarine circulation, so that it becomes difficult to establish a typical, or 

 average, circulation for any estuary. Eliot (1978), for example, found that 

 the Potomac estuary was two-layered and partially mixed during 40% of the time 

 during which it was studied. During the remaining 60% of the time, any one 

 of five flow regimes existed as a result of meteorological influences. 



General Hydrographical Characteristics of Maine Estuaries 



Most Maine estuaries are drowned river valleys, formed as a result of the rise 

 in sea level over the recent geological past. Maine estuaries exhibit all 

 estuarine types, often within the same estuary. For example, the upper part 

 of the Sheepscot estuary is vertically homogeneous; below Wiscasset to Five 

 Island and Townsend Gut it is partially stratified, and below that region it 

 becomes almost a "salt wedge" type (although these boundaries change 

 seasonally and are approximate) . Most Maine estuaries have considerable tidal 

 exchange relative to fresh water flow, so that all experience mixing. The 

 extreme "salt wedge" type is rare. Most estuaries in Maine have a partially 

 mixed character over much of their length, with the upper segment being well 

 mixed to homogeneous. 



Because the circulation of estuaries is principally a function of fresh water 

 flow, topography, and tidal exchange, these parameters have been used by 

 scientists and engineers to characterize and compare estuaries. One of the 

 most widely applied methods is Ketchum's (1951) modified tidal prism method. 

 This method uses topographic and tidal height data and fresh water flow data 

 (available for most major rivers from the USGS). 



The method is described in detail in Dyer (1973; which also includes a 

 comprehensive treatment of general estuarine hydrography); the following 

 description is intended only to indicate the results obtained and some of 

 their limitations. The method hypothetically divides the estuary into a 

 number of segments that are about 2 miles (3.2 km) long in most estuaries in 

 Maine. It then calculates an average salinity, a flushing time, and tidal and 

 nontidal currents for each segment. 



The model operates on the assumption that the contents of each segment are 

 thoroughly mixed by each tide and measures the salinity accordingly. In most 

 Maine estuaries, only a surface layer of 10 to 15 m thickness approximates 

 well mixed conditions and, consequently, the model has been applied only to 

 this layer. Both the segmentation and the salinity distribution change as 

 fresh-water flow changes, so for comparative purposes the 10-year mean flow 

 from USGS water resource data for each estuary was used. These data were 

 adjusted for drainage basin area, since the gaged flow is only for that part 

 of the total drainage basin upstream of the gage (see chapter 6, "The Riverine 

 System") . The predicted salinity distribution is only the average surface 

 salinity and in the Sheepscot estuary, for example, where comparable field 

 data are available, the predicted salinity is usually within 1 ppt of the 

 observed surface salinity. It must be noted though that salinity below the 

 surface is always higher than it is at the surface. Changes in freshwater 

 flow can alter the salinity distribution greatly. Freshwater flow in Maine 

 estuaries varies seasonally between 33% and 300% of the mean. 



The flushing time of a segment of an estuary is the average length of time 

 that water spends in that segment. In the reports of flushing time given 



5-21 



10-80 



