Introduction

In the past decade there has been a concerted research emphasis on the structure, settling, and storage of suspended sediments in freshwater riverine environments.1-5 This body of work has recognized the significance of flocculation and aggregation (terms which are used interchangeably in the literature) in riverine sediment transport processes, and the concomitant implications for the storage of both sediments and

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sediment-associated contaminants. While the mechanisms and factors regulating flocculation, defined as the combination of two or more particles of mineral or organic material to create larger composite particles, have been research interests in the marine literature for decades they were only reported as being significant in natural freshwater systems in the 1990s.6-8 While the process of flocculation increases both the effective size of the particle and modifies its density it has been shown that the propensity for particle settling is influenced more by the particles altered size rather than its density or porosity.5

While the literature details the conditions or mechanisms which promote the flocculation and aggregation of sediments in rivers (increased sediment concentrations, increased collision encounters, decreased shear velocities, high ionic strength, increased bacterial activity, and increased temperatures) there has also been some effort in the literature to subdivide composite particles into two separate populations comprising flocs and aggregates. Different processes and different composite structures have been suggested as a means to differentiate flocs and aggregates. Petticrew and Droppo9 differentiated flocs and aggregates by visual evaluation, with flocs being characterized as irregularly shaped and porous while aggregates appeared opaque and compact. It was postulated by them, and reiterated by Woodward et al.10 that the sources of the two structures were different with the fragile, loosely bound flocs being formed in the water column while aggregates are delivered to the stream from the catchment as robust, compact particles. Petticrew and Droppo9 also considered the fact that the floc structures stored in or on the gravels could be dewatered and potentially become more compact due to biological processes or physical reworking. Droppo et al.11 have proposed a floc cycle for riverine composite particles that suggested a downsizing and consolidation of particles with increased exposure to bed shearing environments, indicating a change in structure over time spent in the river system. While it may be important to determine the source of the composite types it is also of interest to determine the relative abundance of aggregates and flocs in the stream channel and to determine if they behave differently in the context of settling and storage.

The objective of this chapter is to evaluate the morphology, settling behaviour, and characteristics of composite sediments that are transported and stored in a relatively undisturbed productive headwater stream. A case study of a highly productive salmon bearing stream is presented here with both the hydrologically important and biologically important periods of the open water season being investigated over several years. The focus of this chapter is the relationship of these changing environmental conditions with the sediment particle populations in both the water column and gravel storage. The changes in composite particle morphology and their resultant dynamic characteristics (settling rate and densities) were evaluated temporally over a range of open water conditions (May through October) while both the physical environment (suspended sediment load, stream velocities, and shear stresses) and the biological inputs to the stream changed.

Earlier work on O'Ne-eil Creek, reporting on the structure and composition of suspended and gravel stored sediment, indicated that in these biologically active headwater streams the fines (sediments < 63^m) were well flocculated.3,12 The aggregates or flocs exhibited maximum sizes 7 (suspended) to 14 (gravel stored) times greater than the maximum size of the constituent inorganic material comprising the composite structures.3 Petticrew and Droppo9 visually identified different composite structures and observed that these loosely bound flocs and compact aggregates exhibited different settling behaviors and size ranges. As these data were collected during the 1996 die-off of 10,722 sockeye salmon (Oncorhynchus nerka) that had returned to the stream to spawn, follow-up work was undertaken to evaluate the importance of the biological and physical influence of the fish upon the morphometric and dynamic properties of the sediment.

The hydrologically important period in terms of sediment transport in streams of this region is the spring melt which occurs in late May. High flows on the rising limb of this flood event scour and break down the armoured layer in this creek13 mobilizing the supply of channel surface and gravel stored fine sediment. Terrestrial contributions from the floodplain and from the headwater slopes are also observed during this event. Cyclonic summer storms can also generate high intensity rainstorms which act to move sediment into and within the channel. The high flow spring melt events exhibit increased concentrations of suspended fine sediment, increased local shear stresses, and contributions of organic matter which are predominantly terrestrially derived. It was of interest to determine the resultant size, structure, and settling behavior of the composite particle population generated by these interacting set of factors.

Alternately the influence of the dominant biological influence in the stream was of concern, as this stream can have annual sockeye returns of up to 50,000, although on average it receives approximately 10,000 per year.13 The physical effect of the digging of redds, or egg nests dug to about 25 cm into the gravels, is to both modify the surface morphology of the gravel bed and to resuspend the gravel stored fines,14 possibly many times in one spawning season. Following this major physical disturbance of both the gravels and the water column, the fish die in the stream and decay in the late summer low flows. The flux of organic matter to the stream is immense and abrupt15,16 as these salmon spawn in only the lower 2 km of the channel and die in a period of about 10 days, resulting in a high unit area loading of fish breakdown products. Petticrew and Arocena17 observed a chemical signature of salmon flesh in the gravel stored sediments, indicating that either the breakdown products or bacteria with the salmon signature are associated with the fine grained gravel stored aggregates. Given the potential role of organic matter and microbial activity in the generation of composite particles this highly productive stream was seen as a good venue to evaluate the effect of both the supply of organic matter and the physical disturbance of spawning on the structure of flocs and aggregates being transported and stored in streams.

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