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The water samples were returned to the laboratory and processed in a variety of ways. SPM was determined gravimetrically by filtering a known volume of water (commonly 1000 to 4000 ml, depending on concentration) onto preweighed and preashed 47 mm diameter glass fiber filters. A second, smaller volume (100 to 1000 ml) was filtered through preweighed 0.8 /m Millipore cellulose-acetate filters. These were used for determining the disaggregated inorganic grain size distribution also known as the ASPD. The weighed, dried filters were ashed in a low-temperature asher (<60°C) and wet digested with an excess of 35% H2O2 before analysis on a Coulter counter.20 A Coulter Multisizer IIE was used to determine the ASPD. Results are expressed as a volume/volume concentration in ppm and are plotted as smoothed histograms of log concentration versus log diameter.20 The Multis-izer was set to a lower detection limit of 0.63 /m and an upper detection limit of 1200 /m.

The EPSD was determined by filtering smaller volumes of water (10 to 100 ml, depending on the sediment concentration) through 0.45 / m Millipore filters at low levels of suction (< 80 kPa). Care in handing the collected water and in the filtering process ensured minimal disturbance to the aggregate structures. EPSD was measured optically, using a method similar to that of de Boer21 and reported in Biickert.22 These filters were air dried, cut and mounted onto microscope slides to obtain particle morphometrics using the BioQuant OS/2 image analysis system which was connected to a microscope. The filtered particles were counted and characterized for perimeter, area, long axis, equivalent spherical diameter (ESD) and circularity. The population of particles counted per filter was in excess of 1000 and in most cases triplicate filters were analyzed to allow a determination of the variability. To obtain the ESPD, the population's equivalent spherical diameters were grouped into size classes which correspond to the same intervals as the Coulter counter and plotted as volume/volume concentration in parts per million against ESD. The lower limit of the image analysis technique when linked to the microscope is an areal size of 5.4 /m2 and presumably the upper limit would be defined by the area of the filter visible at the given magnification setting which would be in the order of 100,000 /m2. However as the volume of water filtered is often small, because this minimizes overlap of particles on the filter, and because the probability of capturing the larger, rarer flocs is lower due to reduced sample volume, this method tends to artificially restrict the upper limit of the size spectra. For almost all filters analyzed for this study, the maximum aggregate diameter observed was of the order of 400 /m while larger aggregated particles (>700 /m) were observed in the bigger sample volume of the same origin in the settling chamber.

The morphometric parameters collected from the image analysis of the filtered population of aggregates were used to determine the fractal dimension (D) of the populations. D is a measure of the perimeter-area relationship for a set of objects. Collections of natural objects tend to have a perimeter-area (P, A) relationship of A a P2/D .23 Euclidean objects such as squares or circles have a D value of 1. Values of D> 1 indicate that as area increases, perimeter increases at a greater rate.21,24 This means that these larger particles have more edge complexity and are less Euclidean or evenly shaped. Fractal D values were determined from perimeter and area relationships for populations of Altered aggregates as well as particle populations sized and characterized in the settling chamber.

4.2.4 Settling Chamber Measurements

The collection of a larger volume of suspended sediment to determine the fall velocities and densities of suspended sediment structures employed a rectangular plexiglass settling box (1.5 x 0.14 x 0.06 m) with two removable end caps that was built to hold approximately 131 of water. A scale was mounted on the outside back wall of the settling chamber using white adhesive paper which aided in photographing and sizing particles. The settling chamber was aligned into the stream flow such that water and suspended sediment passed through it. When a sample was required the ends were capped and the box carried in a horizontal position to the side of the creek, where it was placed vertically onto a stable platform 20 to 30 cm in front of a 35 mm single lens reflex (SLR) camera mounted on a tripod. After a period of several minutes, during which fluid turbulence decayed, a series of timed photographs were taken. Pairs of sequential images were then projected onto a large surface and examined to identify individual flocs. The particle size, shape, and position in the two images were determined using image analysis packages (Mocha and Bioquant) allowing an estimate of the fall velocity.

In the spring of 1997, the same settling chamber was used to collect suspended sediment samples from the snowmelt flood events in O'Ne-eil Creek. Due to the fast overbank flows at this time the box was lowered and returned to the bridge platform using a winch system. The box was filled and capped by persons standing in the stream. The photographic system employed in the field at this time was a video capture system. A black and white digital camera (a charged-coupled device — CCD), with a resolution of 512 x 512 pixels, was connected to a personal computer running Empix Imaging's Northern Exposure software. This field setup allowed an automated image grabbing system, which recorded the current time (accurate to 10-2 s) on each image. A run of 45 images could be grabbed in just over a 90 sec. The resultant images had individual pixel resolution of 55 ^m ± 10 ^m. The images were then analyzed via a custom-developed22 settling rate measurement program.

Due to colder weather, and shorter day lengths that contributed to poor conditions for outdoor photography, the samples from October 5, 2000 were collected in the field but returned to the laboratory for analysis. In this case up to 12 l of ambient and resuspended sediment-laden water was collected and introduced into the settling chamber for analysis using the SLR camera.

Measurements of particle size and settling velocity for both the SLR and video imaging method allowed for the derivation of particle Reynolds numbers as well as particle density using the equations presented in Namer and Ganczarczyk.25 The lower resolution of particle diameters using these techniques was approximately 150 ^m while the upper limit would be defined by the field of view of the cameras, which given the distance from the settling chamber allows a photographic image of a particle with a long axis in excess of 10,000 jxm.

4.2.5 Infiltration Gravel Bags

On July 13, 2001, twelve infiltration gravel bags were installed in two riffles near the bridge site of O'Ne-eil Creek. A hole approximately 25 cm in depth was dug and the gravels removed were cleaned through a 2 mm sieve. The bags are a modification of the design presented by Lisle and Eads26 and consist of watertight bags, with a maximum volume of 10,000 cm3 clamped onto a 20 cm diameter iron ring. The bag is folded down on itself at the bottom of the hole, while straps attached to the ring are placed along the sides of the hole and left at the gravel-water interface. The cleaned gravel is then returned to the hole, being placed on top of the folded bag and left for a known period of time to accumulate fine sediments in the intergravel spaces. The bag traps were retrieved over a 71-day period following installation. The retrieval dates (cf. Table 4.1) represent (i) the period before the fish return to the river to spawn (July 17), (ii) the early spawn (July 28), (iii) mid-spawn (August 3), (iv) two dates during the major fish die-off (August 12 and August 16), and (v) a sample when there was no visual evidence of live or dead carcasses in the stream, termed post-fish (September 22).

Upon retrieval a lid is placed over the surface gravels between the emergent straps that are pulled up, moving the iron ring and the bag up through the gravels ensuring a minimal loss of fine sediment. The gravels and water collected in the bags were passed through a 2 mm sieve such that the finer sediment was collected in a calibrated bucket. This material was mixed to resuspend all grain sizes, settled for 10 sec to allow the settling of large sands from the top water layer from which a 250 ml subsample was taken for use in the settling chamber. These gravel stored fine sediments were introduced into the settling chamber which was filled with filtered (0.45 ixm) O'Ne-eil Creek water. The CCD digital video method of image collection was used for these samples. Around 100-250 individual particles were tracked for each set of bags, providing size and settling characteristics while larger populations (n = 1000 to 2500) of particles photographed in the settling chamber were used to determine morphometric characteristics of the total population of gravel stored aggregated fine sediment.

4.2.6 Visual Characterization of Aggregate Particles

The images of particles captured in the settling chamber when the SLR camera is used were very clear and distinct such that more detailed structure of individual particles could be evaluated. It was obvious upon viewing the particles for the first time in the year 1995 that some were opaque, appearing to exhibit no open pores while others were a loose and open matrix of material attached together. In some cases the aggregates were a combination of both of these forms. In 1996, we decided to label each particle that we had tracked and for which we had estimated a settling velocity, in order to determine if differences in settling behavior existed between these visual subpopulations. The compact, opaque subset was termed compact particles while the open, loose matrices were called flocs. The combination particles and those which we were unable to define were classed in a group as mixed particles. A fourth subset was added in the year 2000 as visual evaluation of the compact subpopulation indicated that some dense, dark particles had visual indicators that they were organics or parts of organisms. For further clarity these were separated and labeled compact-organic particles.

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