Fish habitat analysis using river flow simulation

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Abstract
Hydrologic and hydraulic data collected at flows between 14.3 and 1.55 m3 s‐1 in a braided section of the Ashley River, Canterbury, New Zealand, were used to test the reliability of the incremental method of habitat analysis using computer simulation. Using data at 14.3 m3 s‐1 as input, water surface profiles, cross‐channel depth and velocity profiles, and values of the habitat index weighted usable area (WUA) for 9 fish species were simulated at the other 4 lower flows, and compared with measured values. Despite the non‐uniformity and wide range of the flows, agreement was remarkably good. Sixty‐seven percent of simulated WUA values were within 20% of the measured values, and the relative changes of WUA with changing discharge were successfully reproduced even when absolute values of WUA were not. The complex nature of flow in a braided river placed limitations on the method; a particular difficulty which warrants further investigation is flow apportionment at branch channel divergence points. As channel complexity increases, a stage is reached where the method which uses hydraulic information collected at multiple flows is simpler and more accurate than the simulation method. Both have a role to play in management of the freshwater resource.

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