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### Drainage basins, channels, and flow characteristics of 
selected streams in central Pennsylvania

#### Professional Paper 282-F

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- Document: **Document**                                
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- Plates:
- **Plate 1**                                
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- **Plate 2**                                
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- **Plate 3**                                
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- Download citation as:  RIS

#### Abstract

The hydraulic, basin, and geologic characteristics of 16 selected streams in 
central Pennsylvania were measured for the purpose of studying the relations 
among these general characteristics and their process of development. The basic 
parameters which were measured include bankfull width and depth, channel slope, 
bed material size and shape, length of stream from drainage divide, and size of 
drainage area. The kinds of bedrock over which the streams flow were noted. In 
these streams the bankfull channel is filled by flows approximating the 2.3-year 
flood. By measuring the breadth and mean depth of the channel, it was possible 
to compute the bankfull mean velocity for each of the 119 sampling stations. 
These data were then used to compute the downstream changes in hydraulic 
geometry of the streams studied. This method has been called an indirect 
computation of the hydraulic geometry. The results obtained by the indirect 
method are similar to those of the direct method of other workers. The basins 
were studied by examining the relations of drainage area, discharge, and length 
of stream from drainage divide. For the streams investigated, excellent 
correlations were found to exist between drainage area and the 2.3-year flood, 
as well as between length of stream from the basin divide and drainage area. 
From these correlations it is possible to predict the discharge for the 2.3-year 
flood at any arbitrary point along the length of the stream. The long, 
intermediate, and short axes of pebbles sampled from the bed of the stream were 
recorded to study both size and sphericity changes along individual streams and 
among the streams studied. No systematic downstream changes in sphericity were 
found. Particle size changes are erratic and show no consistent relation to 
channel slope. Particle size decreases downstream in many streams but remains 
constant or increases in others. Addition of material by tributaries is one 
factor affecting particle size and another is the parent material. Wear does not 
appear to account for some of the changes noted in particle size in a downstream 
direction. Comparison with laboratory studies indicates that at least in some 
streams the downstream decrease in size is much greater than would be expected 
from wear alone. The type of bedrock underlying the channels included in this 
study appears to affect both channel slope and particle size. For a given length 
of stream, a stream channel underlain by sandstone tends to have a steeper slope 
and larger bed material than channels underlain by shale or limestone. Hence, a 
stream which heads in sandstone and ends in limestone tends to have a more rapid 
decrease in slope and particle size than a stream heading in limestone and 
ending in sandstone. The association of steep slopes and small particles for 
limestone channels implies that slope and particle size may show a vague 
correlation between lithologic groups although no correlation may exist within a 
given lithologic type. In addition to the effect of bedrock on slope and 
particle size, there is some evidence that channels in limestone or dolomite 
have a slightly smaller cross section at bankfull stage than channels in shale 
or sandstone. Near the headwaters of many of these streams, a deposit of 
periglacial rubble affects the slope and bed material size. Some of the debris 
contains residual boulders which are too large to be moved by ordinary floods 
and, therefore, impose larger particle sizes in the bed of the stream. The 
addition of this very coarse debris to the bed material is another example of 
the influence of geologic factors on stream channels even though the channel 
consists of unconsolidated debris instead of bedrock. The influence of geologic 
factors noted in selected streams in central Pennsylvania may not be directly 
applicable to areas other than the Appalachian Mountains, but the general 
process is no doubt similar in most areas. In large alluvial valleys bedrock 
cannot be much of an influencing factor; yet large, thick alluvial deposits and 
terraces are in a sense "bedrock" materials upon which the stream works to form 
the landscape.

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