Graduate Thesis Or Dissertation
 

Factors influencing pool morphology in Oregon coastal streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/js956j09k

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  • Pool morphology was surveyed in 19 stream sections within the central Oregon Coast Range. Pool locations, sizes, spacings, numbers, and factors affecting pool formation were determined for each stream section. All sections were underlain by sedimentary rocks, had drainage areas ranging from 1.3 to 17.3 km², and had average water surface slopes from 0.5 to 5.6%. Stream sections were divided into two categories: (1) low timber harvest (<20% of watershed area harvested) and (2) high timber harvest (>45% of watershed area harvested). A "Rapid Bed Profile" (REP) technique was developed to measure residual pool characteristics in each stream section. The REP technique is a survey method that requires only thalweg depths and the average reach gradient. The technique was effective for classifying pools since it is objective, independent of flow, accurate, and time-efficient. Residual pool size characteristics (e.g., volume) for the low timber harvest stream sections were positively correlated to a power function of drainage area. Stream sections with beaver dams, especially those with at least 10% of their reach length in beaver-caused pools, typically had larger residual pools. Pool size characteristics for high timber harvest stream sections were not different from low timber harvest stream sections. The average spacing between residual pools was positively correlated to a power function of drainage area for the low timber harvest stream sections (a negative correlation was found between the number of pools and drainage area). High timber harvest stream sections may be associated with an increased spacing and a decreased number of pools for larger watersheds (i.e., greater than 8 km²). However, the potential effects of previous large storms, changes in timber management practices, and/or the small number of streams surveyed precluded a definitive conclusion. The frequency of occurrence of pool forming processes (e.g., plunge, deflection) was correlated with average water surface slope for the low timber harvest stream sections. The percentage of plunge and impoundment processes increased as water surface slope increased while the percentage of deflection and underflow processes decreased. Two high timber harvest streams had a higher percentage of plunge pools than expected based on the relationships established for the low timber harvest streams. The frequency of occurrence of wood and boulder pool forming elements was correlated with an index of stream power (drainage area times average water slope) for the low timber harvest stream sections. As the stream power index increased, the relative frequency of wood-formed pools decreased while boulder-formed pools increased. Wood and boulder combined, generally, made up 80% of the pool forming elements. The frequency of occurrence of pool forming elements was not different between low and high timber harvest stream sections.
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