Long-term management of critical water resources infrastructure needs to incorporate projected changes to environmental conditions. Reservoirs form the heart of water resource assets. Long-range plans for the repair, replacement, maintenance and renovation of these facilities depend on accurate projections of reservoir pool elevations. Environmental conditions, in turn, dictate the magnitude and timing of inflows and outflows from reservoirs, and thus the resulting water surface elevation. This lesson explores the factors that affect reservoir pool elevation and the considerations and challenges that changing reservoir pool elevations pose to managing existing water resources infrastructure.