The third type of stream has developed in regions 1 through 4, in the marine 

 clay valleys and low plains. Stream courses are variable here, sometimes 

 controlled by glacial topography, regional slope and, where downcutting has 

 intersected the bedrock surface, by the bedrock structural grain (from NE to 

 SW). 



Landform and chemical composition of bedrock exert a major influence on stream 

 hydrology. Landform (e.g., relief, elevation, and soil development) 

 determines the shape and size of the drainage basin and modifies the amount 

 and pattern of stream flow in conjunction with precipitation. Chemical 

 composition of bedrock and soils is a major influence on water chemistry. 

 Bodies of water in the coastal zone with their drainage basins (if they are 

 larger than 25 acres) are shown in atlas map 3. 



Landform. Among the effects of geology on stream hydrology perhaps the 

 most basic is that of regional topography on the size of drainage basins. 

 Larger drainage basins not only collect more water for a given regional 

 rainfall but, because of the local and scattered nature of storms, provide a 

 more regular input of water from precipitation than smaller basins. 

 Topography also influences the developmental pattern of stream channels. 

 This, in turn, influences flow patterns, since highly-branched channel systems 

 tend to have a more regular flow than less-branched systems (Beaumont 1975). 

 Irregular or variable flow increases a stream's vulnerability to erosion and 

 affects behavior of some stream organisms (see "Precipitation," above). 



Topography, particularly slope, and the geological history of an area, 

 especially glaciation history, have a strong influence on the way water from 

 precipitation enters a stream. In areas with steep slopes and a history of 

 glaciation, (e.g., the headwaters or source areas of many Maine streams) soil 

 development generally is limited and bedrock is present near the surface. 

 Under these conditions percolation of water through the substrate into 

 groundwater is minimal and most precipitation reaches the stream by surface 

 runoff or through-flow (Beaumont 1975). In downstream reaches, where slopes 

 are more moderate and peat and soil development is more extensive, groundwater 

 discharge is greater (Beaumont 1975). Current velocity, which is dependent on 

 slope, channel geometry, and substrate roughness, tends to decrease along the 

 upstream to downstream gradient. Current velocity is an important factor in 

 determining substrate type (high velocity results in large particles), amount 

 of erosion (high velocity increases erosion) , stream load (high velocity 

 transports more material), and degree of mixing within the water column and 

 between the water and the air (high velocity creates greater turbulence and 

 mixing) . These phenomena are significant biologically and contribute to the 

 development of biological communities along the length of a stream. 



Bedrock composition . Chemical composition of the bedrock and soils 

 within the drainage basin is normally the most dominant factor in determining 

 water chemistry within a stream system, although airborne pollutants, runoff 

 pollutants from developed areas, and pollutants dumped directly into the water 

 may be dominant factors in some areas. Calcareous bedrock (e.g., limestone) 

 and soils derived from it usually are associated with streams that have high 

 levels of inorganic plant nutrients, reflected in high conductances and 

 alkalinities . Calcareous deposits within the coastal zone are extremely 

 limited and, therefore, geologic contributions of nutrient ions to Maine 

 rivers are relatively slight. Peaty soils, common in Maine lowlands, may 



6-13 



10-80 



