basin of any of the riverine systems in Maine, followed by the Kennebec (5780 

 sq miles; 14,970 sq km), Androscoggin (3450 sq miles; 8936 sq km) and St. 

 Croix (1635 sq miles; 4235 sq km). As is to be expected, the Penobscot River 

 also has the largest average discharge (11,817 cu feet/sec; 335 cu m/sec) 

 followed by the Kennebec (4436 cu feet/sec; 126 cu m/sec), Androscoggin (3707 

 cu feet/sec; 105 cu m/sec) and St. Croix (2692 cu feet/sec; 76 cu m/sec). 

 Other rivers in coastal Maine have smaller drainage basins (<700 sq miles; 

 813 sq km) and an average flow of <1000 cu feet/sec (28 cu m/sec). Bodies of 

 water with drainage basins (if they are larger than 25 acres) are shown in 

 atlas map 3. 



Riverine systems of coastal Maine do not exhibit an orderly progression of 

 subsystems environments from mouth to head (tidal to intermittent subsystems). 

 Only the larger rivers of the characterization area (Penobscot, Kennebec, 

 Androscoggin, and St. Croix) exhibit any type of progression and within these 

 only the two lower subsystems, tidal and lower perennial, are present within 

 the coastal zone. Smaller streams may not contain the tidal or intermittent 

 habitats. Small streams account for the majority of the acreage of upper 

 perennial riverine habitat in coastal Maine. Many other rivers in the system 

 originate in lakes and ponds and enter the estuarine system over bedrock falls 

 that exceed the vertical tidal range height. 



The four subsystems of the riverine system are defined on the basis of river 

 morphology and geology. A compilation of the habitat types found within each 

 named stream in coastal Maine is given in appendix A (see also atlas map 1). 

 The tidal subsystem is that portion of the river streambed where daily water 

 level and flow fluctuates under the influence of tides. Flow may reverse in 

 this subsystem during periods of spring tides and low river discharge. The 

 tidal subsystem in Maine coastal riverine systems may be as long as 19 miles 

 (30 km), as it is in the Kennebec River, or absent entirely. The Penobscot 

 River is the other river having a substantial section of freshwater tidal 

 habitat. The river channel gradient of the tidal subsystem is the lowest 

 gradient within the riverine system. Gradients are commonly as low as .015% 

 (Kennebec River from Augusta to Bowdoinham) , meaning that over a given 

 distance (e.g., 10 miles), the gradient or slope of land is .015% of that 

 distance (e.g., .0015). 



The morphology of the tidal subsystem is variable and dependent upon a number 

 of factors, including river discharge, channel slope, sediment load, and the 

 surficial or bedrock geology of the lower river basin. The larger river 

 basins contain relatively wide, straight river valleys cut into bedrock or 

 surficial deposits. These valleys, from 99 to 216 feet (30 to 65 m) deep and 

 up to 1.6 miles (2.5 km) wide, were cut during post-glacial rebounding (see 

 "Geology," in chapter 2). The riverine tidal basins originated from the lower 

 perennial subsystem prior to its drowning by a rise in sea-level approximately 

 7000 years ago (Schnitker 1974) . With the reduction in the river channel 

 gradient that resulted from this drowning of the lower perennial subsystem, 

 sedimentation occurred within the channel sections. Deposition on channel 

 bottoms formed channel beach/bars, flats, and emergent wetlands. Upson and 

 Spencer (1964) found up to 13.3 yards (12 m) of estuarine-riverine sediments 

 below the active channel bottom in the Kennebec River at Bath and <3 feet (1 

 m) at Augusta (figure 6-2). Their investigations in the Fore, Sheepscot, 

 Penobscot, and St. Croix Rivers also indicate channel aggradation in the tidal 

 subsystem portions of these waterways. 



6-4 



