During the daylight hours, the shallow regions of a reservoir sidearm absorb more heat per unit volume than the deeper parts, leading to a nett horizontal pressure gradient that drives a circulation in the sidearm. The spin up time for a typical sidearm is of the order of or longer than a day, implying that the flow is intrinsically unsteady. In this paper, the unsteady daytime circulation and temperature structure in a reservoir sidearm is modelled by the natural convection of a fluid contained in a 2-D, triangular cavity. The absorption of solar radiation that drives the flow is modelled by Beer’s law and a heat flux bottom boundary condition formulated from the amount of heat that penetrates the entire local depth. Asymptotic solutions of the resulting equations are found and these reproduce some of the observed features of the daytime circulation in a reservoir sidearm.