Abstract:
This study was conducted at portions of Trap Bay Creek, a medium-sized third-order stream on Chichagof Island, Southeast Alaska, to
1) quantify short-term sediment transport and channel morphology changes, 2) relate measured sediment transport rates to the major hydrologic parameters that appeared to determine the mechanisms of sediment transport, and 3) evaluate how bedload transport can influence channel morphology. Morphologic characteristics were evaluated by
means of plani-metric surveys and cross-sectional measurements made in July and August of 1980 and August of 1981. Bedload and suspended sediment data were collected during the Fall of 1980 along with data
on streamflow and precipitation. Morphologic evaulations indicated that the stream is a dynamic system and that it appeared to be widening and aggrading during 1980-81. Large organic debris, especially fallen trees, are important in stream morphology, especially above the zone of tidal influence. The tides, as well as human activity, probably contributed to recent morphological changes in lower reaches of the channel. Problems involved in processing suspended sediment samples resulted in a limited amount of suspended sediment data. However, data that were collected from one point on the stream indicated that suspended sediment concentrations were low, usually less than 5 mg·1⁻¹, and did not exceed 90 mg·1⁻¹ during an approximate 2 to 5-year return interval storm event. Under average storm conditions, therefore, suspended sediment transport appears to be supply limited and constitutes
a small portion of total sediment transport.
Bedload sediment samples were collected from a short pool-riffle study reach during a total of ten storms with streaflow ranging from 0.01 to 1.26 m³·s⁻¹·km⁻². Bedload sediment transport ranged from 3.9 to 4400 kg·hr⁻¹, with peak transport rates occurring during peak streamflow. Regression relationships were developed between bedload transport, gravel-sized inorganic bedload transport, coarse particulate organic matter transport, two particle size diameter classes (D₅₀ and D₉₀), and stream discharge during the ten storms. This analysis revealed that total inorganic- bedload transport was more strongly related to discharge than was transport of the large size category, coarse
particulate organic matter transport tended to be more strongly related to streamflow than bedload discharge, and that neither of the particle size diameters had any consistent relationship to streaflow. Bedload transport during the ten storms was further evaluated in terms of the sampling sites that were used, i.e. riffles above and below a depositional area approximately 20 m in length. Transport tended to be greater, in terms of amount transported, at the upper riffle for most of the storm events. The opposite was true during the largest storm of the season and a storm which occurred a week later. It may be that bedload sediment is transported past the upper riffle by lesser magnitude events and is temporarily stored in the pool. Transport out of the pool requires events of greater magnitude. Supply limitations also appear to determine bedload transport in Trap Bay Creek.
