This paper describes a qualitative model of the functioning of salmon redds, places where salmon reproduction occurs in the rivers. For this, we used Qsim, which appeared adequate for representing available qualitative knowledge of hydroecology experts. Since the number of relevant variables was relatively large, it appeared necessary to decompose the model into two parts, corresponding to processes occurring at separate time-scales. A qualitative clock allows us to submit the simulation of the development stage of salmon to the calculation of the moving sum of average daily temperatures, according to the clock ticks and a water temperature profile set by the user. There-fore, this introduces some way of real-time dating and duration in a purely qualitative model. Simulating both sub-models by means of alternate QDE transitions, allows us to generate the evolutions of variables of interest, such as the mortality rates according to several factors (e.g. lack of oxygen), under various rainfall scenarios. This work is part of a project aiming at assessing the impact of the environment on salmon populations dynamics, by the use of models of processes acting at different levels : catchment (e.g. erosion), river (e.g. sediment transport and deposition), and redds (e.g. fine particles sealing off). Only the latter level is dealt with in this paper.