In many mountain areas in Southwest China, cascaded run-of-the-river hydropower plants and photovoltaic plants with local loads are connected by ac network comprise fully renewable microgrids. These microgrids are connected to the external large power systems via long-distance transmission lines in normal operation mode. However, when the transmission line's planned or emergency outage occurs, the microgrids are forced to operate in islanded mode. Considering the spatialoral coupling and limited energy storage of the cascaded hydropower plants and the non-Gaussian uncertainty of solar power generation, the frequency control and intra-day energy management, i.e., power-sharing between the cascaded plants, can be challenging. In this paper, a multi-timescale control method is proposed to coordinate the secondary frequency control and intraday energy management. Low-dimensional polynomial models of the expectation and variance of the response of the secondary frequency control over the short timescale are obtained first. Incorporating this model and a dynamic river model, the long timescale energy management problem is formed as chance-constrained programming. The expectational cost function defined over the multiple timescales is minimized by solving this model in a receding-horizon manner. The proposed method is verified by simulations of a real-life hydro-solar system in Sichuan Province, China.