Debris flows, debris floods and floods in mountainous areas are responsible for loss of life and damage to infrastructure, making it important to recognize these hazards in the early stage of planning land developments. Detailed terrain information is seldom available and basic watershed morphometrics must be used for hazard identification. An existing model uses watershed area and relief (the Melton ratio) to differentiate watersheds prone to flooding from those subject to debris flows and debris floods. However, the hazards related to debris flows and debris floods are not the same, requiring further differentiation. Here, we demonstrate that a model using watershed length combined with the Melton ratio can be used to differentiate debris-flow and debris-flood prone watersheds. This model was tested on 65 alluvial and colluvial fans in west central British Columbia, Canada, that were examined in the field. The model correctly identified 92% of the debris-flow, 83% of the debris-flood, and 88% of the flood watersheds. With adaptation for different regional conditions, the use of basic watershed morphometrics could assist land managers, scientists, and engineers with the identification of hydrogeomorphic hazards on fans elsewhere.

Riparian vegetation and hydrogeomorphic processes are intimately connected parts of upland catchment and fan environments. Trees, shrubs and grasses and hydrogeomorphic processes interact and depend on each other in complex ways on the hillslopes, channels and cone-shaped fans of torrential watersheds. While the presence and density of vegetation have a profound influence on hydrogeomorphic processes, the occurrence of the latter will also exert control on the presence, vitality, species, and age distribution of vegetation. This contribution aims at providing a review of foundational and more recent work on the dependencies and interactions between hydrogeomorphic processes and vegetation. In particular, we address the role of vegetation in the initiation of hydrogeomorphic processes and its impact on stream morphology as well as immediate and long-term effects of hydrogeomorphic disturbance on vegetation.

Alluvial and colluvial fans with forest cover are common in the valleys of west central British Columbia, Canada. Given the low population density of the region, most of these fans are uninhabited and the primary land use is forestry. The fans are desirable for timber harvesting due to the combination of the relatively easy access to their valley-bottom locations and their high-quality timber. However, they are also sites influenced by debris flows, debris floods and floods, and the interaction between conventional forest practices and these natural hydrogeomorphic processes has led to substantial financial costs and disturbance to forests and stream channels. Basic watershed morphometrics can be used to predict the dominant hydrogeomorphic process influencing forested fans. The hydrogeomorphically active zones of forested fans have characteristic site and stand features, and are referred to as the hydrogeomorphic riparian zone. Features within these zones can be used to determine the frequency and disturbance extent of hydrogeomorphic events. Appropriate management strategies can be developed to limit the effect of forestry activities on natural hydrogeomorphic processes.

Forested fans are often crossed by roads and their high-volume stands are attractive for harvesting. Gentle slopes of fans belie the fact that hydrogeomorphic hazards can be present. Fans can be the run out zones for debris flows and they can be subject to floods and debris floods. This study assessed the effect of natural hydrogeomorphic processes on forest practices that were undertaken on 55 fans in west central British Columbia. Forest practices aggravated these processes on 41 (74%) fans, leading to increased erosion and destabilization of fan surfaces and stream channels. Identification of hydrogeomorphic hazards is needed to avoid the adverse effects of forestry practices on fans.Keywords: forested fans, forestry on fans, hydrogeomorphic processes, forest practices on fans, forest practices Les cônes d'alluvion boisés sont souvent parcourus de routes et leurs peuplements à fort volume de bois sont attrayants pour l'exploitation forestière. Les pentes douces des cônes d'alluvion cachent le fait que des accidents hydrogéomorphologiques puissent survenir. Les cônes d'alluvion peuvent être des zones de débordement pour les écoulements de débris et peuvent subir des inondations et déverse-ments de débris. Cette étude a évalué l'effet des processus hydrogéomorphologiques naturels sur les pratiques forestières qui ont été entreprises sur 55 cônes d'alluvion dans le centre-ouest de la Colombie-Britannique. Les pratiques forestières ont aggravé ces processus sur 41 (74 %) des cônes d'alluvion, menant à une érosion accrue et une déstabilisation de la surface des cônes d'alluvion et des canaux des cours d'eau. L'identification des risques hydrogéomorphologiques est requise afin d'éviter les effets négatifs des pratiques forestières sur les cônes d'alluvion.

Pressures on water resources due to changing climate, increasing demands, and enhanced recognition of environmental flow needs result in the need for hydrology information to support informed water allocation decisions. However, the absence of hydrometric measurements and limited access to hydrology information in many areas impairs water allocation decision-making. This paper describes a water balance-based modeling approach and an innovative web-based decision-support hydrology tool developed to address this need. Using high-resolution climate, vegetation, and watershed data, a simple gridded water balance model, adjusted to account for locational variability, was developed and calibrated against gauged watersheds, to model mean annual runoff. Mean monthly runoff was modeled empirically, using multivariate regression. The modeled annual runoff results are within 20% of the observed mean annual discharge for 78% of the calibration watersheds, with a mean absolute error of 16%. Modeled monthly runoff corresponds well to observed monthly runoff, with a median Nash-Sutcliffe statistic of 0.92 and a median Spearman rank correlation statistic of 0.98. Monthly and annual flow estimates produced from the model are incorporated into a map-and watershed-based decision-support system referred to as the Northeast Water Tool, to provide critical information to decision makers and others on natural water supply, existing allocations, and the needs of the environment.(KEY TERMS: surface water hydrology; computational methods; decision-support systems; Northeast Water Tool.)