Where it intersects the coastline it forms a deep, 

 narrow embayment superficially resembling a 

 fjord. The valley, however, is river cut rather 

 than glacier cut (Johnson, 1925) and has no sill 

 at its seaward end. It is a typical submerged 

 or "drowned" river valley. The size of this 

 portion of the estuary relative to the size of the 

 river itself is so large that it is not, strictly 

 speaking, an estuarine environment. For the 

 purposes of this discussion, therefore, it will 

 be referred to as the lower estuary and distin- 

 guished from the upper or true estuary, which 

 occupies a shallow valley about five miles long 

 from the river mouth to the upper end of the 

 lower estuary. The town of Wiscasset, Maine 

 and the U.S. Route 1 highway bridge form a 

 convenient dividing line between the two. 



The lower estuary ranges from about 60 

 meters in depth at its seaward end to about 20 

 meters at its upper end. For the most part its 

 shores are steep and rocky. The upper estuary 

 is much shallower, with a channel from 1 to 10 

 meters in depth at mean low water and bordered 

 by extensive mud flats and salt marshes. About 

 midway along this upper estuary at Sheepscot 

 Village is a constriction between two headlands 

 where a transverse ledge of rock tends to hold 

 back the water above it, creating a waterfall on 

 the ebbing tide . At one time a mill dam was 

 erected at this site. There are other constric- 

 tions at several places in the estuary where 

 currents are rapid but without falls . There are 

 two major tributary branches to the upper estu- 

 ary: Marsh River and Dyer River . Both are 

 largely tidal but are fed by streams. 



As can be seen in fig. 1, the lower estuary 

 is a complex affair of interconnecting channels 

 including one, the Sasanoa River, which connects 

 it directly with the nearby Kennebec River. 



Hydrography 



In contrast to the lower estuary of the 

 Sheepscot, the upper estuary has these char- 

 acteristic features: shallower water, greater 

 tidal exchange ratio, lower and more variable 

 salinity, and a wider range of temperature . The 

 upper estuary shows the effects of the river very 

 noticeably at all times of the year . The river 

 discharge varies from about 20 to over 1000 cub- 

 ic feet per second during the course of a year. 



being greatest at the time of the spring thaw in 

 April or after heavy rains, and lowest during 

 August and September . The salinity of the 

 estuary varies accordingly. 



Tidal variation in salinity depends on 

 the stage of the river and the particular locality, 

 but fluctuations between high and low tides often 

 exceed 15 o/oo in much of the upper estuary. 

 Seasonally, at a given location and tidal stage, 

 salinity varies by about 15 to 20 o/oo, as can 

 be seen from Station 13 in fig. 3 . 



In comparison, the lower estuary is 

 characterized by: deeper water, smaller tidal 

 exchange ratio, higher and less variable salinity, 

 and a narrower range of temperatures. Dilution 

 by the Sheepscot River is rapidly attenuated be- 

 low Wiscasset and is hardly distinguishable five 

 miles south of that town where, due to the nar- 

 rowness of the basin, swift currents mix the 

 diluted surface water with the highly saline deeper 

 layers. The mixed water extends southward to 

 the region opposite the entrance of the Sasanoa 

 River near Station 3 where the surface water is 

 again diluted by an influx of low salinity water 

 from the Kennebec . The area thus influenced 

 extends southward along the western shore of 

 the lower estuary to the Gulf of Maine . 



Below a depth of ten meters water of 

 rather uniformly high salinity extends the entire 

 length of the lower estuary. Fig. 2 shows the 

 vertical distributions of salinity on a longitudinal 

 section of the estuary from the Gulf of Maine to 

 Wiscasset as it appeared on a typical spring day 

 (showing the effect of the spring runoff) and on 

 a typical summer day. Seasonal salinity trends 

 are shown in fig . 3 . 



There are pronounced horizontal and 

 vertical temperature gradients in the estuary, 

 particularly in summer . There are also seasonal 

 and tidal variations, which are strongest in the 

 inner estuary or in other places where the water 

 is shallow and protected. The greatest thermal 

 stratification occurs from mid- April to Septem- 

 ber, although during this period intermittent 

 mixing by waves, wind and current may partially 

 dissipate it. After September the mixing over- 

 comes any surface warming and temperatures 

 are nearly uniform at all depths, although during 

 the coldest part of the winter the surface water 

 may be somewhat cooler than tJiat at the bottom . 



