What should these rivers look like? Historical range of variability and human impacts in the Colorado Front Range, USA

Historical range of variability (HRV) describes the range of temporal and spatial variations in river variables such as flow regime or channel planform prior to intensive human alteration of the ecosystem. In mountainous river networks, HRV is most usefully applied to spatially differentiated geomorphic process domains with distinctive form and process. Using the Colorado Front Range as an example, three examples of how knowledge of HRV can assist river management and restoration are discussed. The examples involve instream wood load and channel morphology, beaver colonies and valley-bottom form and process, and flow thresholds in regulated rivers. The question of what a river should look like - that is, what range of process and form the river included prior to intensive human alteration - can be addressed by (i) placing the river within a process domain, (ii) establishing correlations between form parameters that can be remotely sensed and reach-scale process and form, so that the spatial extent, connectivity, and rarity of process domains within a river network or a region can be quickly assessed, (iii) inferring characteristics of the river prior to intensive alteration by documenting characteristics of the least altered reference rivers and by using proxy indicators of pre-alteration conditions, and (iv) establishing process thresholds that must be exceeded to maintain form (e. g. flow thresholds to mobilize bed sediment). Once this context has been established, resource managers can better evaluate the options for restoring altered riverine form and function. Copyright (C) 2011 John Wiley & Sons, Ltd.