The NWI includes the following classes within the tidal subsystem in coastal 

 Maine: open waters, unconsolidated bottom, flat, rocky shore, beach/bar, and 

 nonpersistent emergent wetland (figure 6-1). While most rivers exhibit at 

 least one of these classes within their tidal sections, only the larger ones 

 contain all of the classes. Merrymeeting Bay (region 2), the confluence of 

 the tidal reaches of the Androscoggin, Kennebec, Cathance, Abbagadasset , and 

 Muddy rivers, has all of the above classes. The main streambeds of the 

 Kennebec and Androscoggin rivers are open water, with unconsolidated or rock 

 bottoms. Unconsolidated bottoms probably erode during spring freshet peaks 

 and aggrade as discharge decreases. Most bottom sediment ranges probably from 

 silt to medium sand. 



Bordering the tidal streambeds are partially subaqueous bars, including mid- 

 channel, lateral (parallel alongside) and point (sticking out from the land) 

 bars of relatively clean fine sand, or partially submerged mud-flats that rise 

 gently in slope to emergent wetlands covered with nonpersistent fresh-water 

 aquatic plants. Flat and bar environments are submerged constantly during 

 periods of high river discharge but may be uncovered daily during low flows 

 and low tides. Beaches and rocky shores also comprise shoreline areas of the 

 tidal subsystems of larger rivers. The origin and composition of sediment in 

 all tidal classes is discussed in chapter 5, "The Estuarine System." Tidal 

 subsystems of smaller rivers and streams generally exhibit a zonation of 

 classes from open water stream bed to flat to emergent wetland or rocky 

 shoreline . 



The lower and upper perennial subsystems of rivers and streams within the 

 characterization area may be juxtaposed or intermixed (atlas map 1), depending 

 upon the geologic substrate over which the channel flows. The lower perennial 

 subsystem is that part of the stream where the channel gradient is low; flow 

 is slow; the channel substrate consists predominantly of sand and mud; and the 

 floodplain is relatively wide. The upper perennial subsystem is that channel 

 segment where the channel gradient is high; flow is fast; the channel 

 substrate consists of sand, gravel, and cobbles; and little or no floodplain 

 is present. The larger river systems (Androscoggin, Kennebec, Penobscot, and 

 St. Croix) all exhibit typical lower perennial subsystems above the tidal 

 stretches throughout their courses within the characterization area. The 

 floodplains of these are not very wide and some channel sections may flow over 

 bedrock or contain substrates of gravel. However, substrates are 

 predominantly depositional and vegetated lateral and point bars have built up 

 on the inside of channel bends. Smaller rivers such as the Dennys , Machias, 

 East Machias, Presumpscot , St. George, Fore, Sheepscot, and Narraguagus have 

 sections of lower perennial channels separated by upper perennial segments. 



Lower perennial segments flow through low terrains such as broad valley 

 bottoms, and glaciated terrains whose drainage has been blocked locally by 

 moraines or other ice-contact deposits. Channel segments are generally 

 sinuous, meandering with local subaqueous bars and lateral nonpersistent 

 emergent wetlands, or may flow through palustrine wetlands, with the channel 

 bounded on three sides by palustrine deposits. Upper perennial reaches are 

 generally straight and can be broken down into pools and riffles. Pools are 

 relatively low-flow segments of a reach where the depth of the channel is 

 greater than in other reaches. The channel bottom still may consist of sand, 

 gravel, cobbles, and boulders deposited during periods of high flow; these 

 sediments may be covered with mud, which settles from the water column during 



6-9 



10-80 



