Publication (Journal article): Spatial variation in the force required to initiate rock movement in 4 upland streams: implications for estimating disturbance frequencies

Publication Type:	Refereed journal article
Publication Name:	Spatial variation in the force required to initiate rock movement in 4 upland streams: implications for estimating disturbance frequencies

Reference Information

Downes, B.J., Glaister, A. and Lake, P.S. (1997) Spatial variation in the force required to initiate rock movement in 4 upland streams: implications for estimating disturbance frequencies. Journal of the North American Benthological Society, Vol 16, pp 203-220.

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ISBN:
8873593

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Key words: scale, disturbance, rock movement, shear stress, spatial variation, critical force - rock size - bed packing, FLOW-COMPETENCE; HABITAT TEMPLET; COMMUNITY STRUCTURE; SHEAR-STRESS; TRANSPORT; BED; PREDICTABILITY; HETEROGENEITY; HEMISPHERES; VARIABILITY

Abstract:

Lotic models of disturbance generated by floods and spates suffer from 2 main shortcomings: a lack of knowledge regarding the appropriate spatial scale at which to apply models and a poor understanding of the relationship between discharge sizes and actual disturbance frequencies and intensities. Here, we examine the spatial variability in the forces needed to shift rocks and the utility of hydraulic equations that predict critical shear stresses (tau(c)), which are sometimes used to infer disturbance frequencies in streams. We used spring balances to measure directly the forces needed (F-c) to move rocks up and out of the stream bed in 4 upland streams (Acheron River, Taggerty/Steavenson rivers, Connelly Creek, and Little River) in southeastern Australia. We measured 25 rocks at each of 32 sites overall, with sites distributed in a nested design: sites were paired in 2nd, 3rd, upper 4th, and lower 4th orders on each river. For each rock, we determined whether it was wedged into place by surrounding rocks, estimated percentage burial in fine sediments, and measured rock size and ambient water velocity and depth. Nested analyses of variance indicated that F, and its correlates varied most between rivers and greatly between rocks within individual sites; the spatial scales of stream order and site contributed little explanatory power. Hierarchical, log-linear modelling showed that both rock size and bed packing varied systematically between rivers, with the Little and Taggerty/Steavenson rivers having relatively large rocks that were often packed into the bed, whereas Connelly Creek and the Acheron River had many relatively-small rocks lying loosely on top of the bed. A river-by-river analysis showed that Values of F-c were related highly to rock sizes but that the nature of the relationships differed greatly between packed-in rocks and those lying on top of the bed and also varied between rivers. The Little and Taggerty/Steavenson rivers were similar to each other but both differed from the Acheron River and Connelly Creek, which differed from each other. Our estimates of F-c suggest that an oft used approximation, which equates tau(c) directly with rock sizes in mm, and the commonly-used equations from which the approximation is derived, are likely to produce poor estimates of tau(c); these poor estimates would cause equally poor estimates of likely disturbance frequencies. The application by ecologists of reach-level hydraulic equations to estimate shear stresses and the sizes and numbers of rocks moved by floods and spates could be flawed by a focus on inappropriate spatial scales. Our data suggest that Variation in likely disturbance frequencies between rocks within individual sites might be of a similar magnitude to variability between different rivers. We argue that spatial variation in stream systems need not be organized in the top-to-bottom hierarchical models that have been recently promoted for rivers.

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