The Effect of Flow Regulation on the Distribution and Dynamics of Channel Geomorphic Units (CGUs) and Implications for Marble Trout (Salmo marmoratus) Spawning Habitat in the Soča River, Slovenia.

A river channel survey was completed along three reaches (totalling 14.3km), i.e. an unregulated stretch and two regulated reaches (with reduced flows) of the Soča River to assess the spatial pattern of CGU type, size, hydraulics and distribution. In addition, one regulated reach was re-surveyed at different discharges to investigate the dynamics of CGUs and their relationship with flow. CGUs were classified and mapped using visual assessment and physical measurements of the hydraulic characteristics (velocity and depth) in each CGU. The effect of flow regulation on the hydraulic character of the river becomes apparent by highlighting differences in the types of CGUs present between the regulated and unregulated reaches. Reduced flows from river regulation also significantly reduces the size of CGUs, alters their hydraulic character, and affects the longitudinal distribution of types by creating greater habitat fragmentation. Hydraulic preferences for spawning habitat of marble trout (Salmo marmoratus) were obtained from previous research. The hydraulic character of CGUs were analysed at different discharges and combined with the hydraulic preferences of the species to evaluate the impact of flow regulation on habitat availability. Analysis shows that intermediate measured flow provides increased spawning habitat availability in the chosen reach for this target species.