In hydro-dependent power systems, water availability varies due to droughts and climate variability, increasing the risk of supply shortages. We present an integrated framework for planning generation and transmission expansion under hydrological uncertainty. The approach uses Monte Carlo simulations to estimate hydropower availability factors and a multi-stage MINLP model that integrates reservoir operation and transmission line reconductoring. It is applied to a 24-bus equivalent system over the 2025–2040 period and evaluated under three scenarios with 19,200 simulations. The full renewable scenario reduces total system cost and unserved power