Run-of-the-river hydroelectric facilities are designed to generate electricity without significantly disrupting the flow of the river. They typically don't have large reservoirs to store water. Instead, a portion of the river's water is diverted into a channel or pipeline, leading to a turbine before returning to the river. Reservoir-based dams, on the other hand, rely on large volumes of stored water to produce power on demand. While reservoir-based systems can offer greater capacity and storage advantages, run-of-the-river systems are viewed as more environmentally friendly due to lesser land inundation and reduced ecological disruption.

Pumped storage is a type of hydroelectric power generation used to balance supply and demand on the electrical grid. During periods of low electricity demand or excess supply (like when there's abundant wind or solar power), excess electricity is used to pump water from a lower reservoir to an upper one, storing energy in the form of gravitational potential energy. During periods of high demand, this water is released to flow back down, passing through turbines to generate electricity. In essence, pumped storage systems act like rechargeable batteries, storing energy during periods of excess and releasing it during deficits.

Maintenance of hydroelectric dams is crucial for their long-term operation and safety. Challenges include sediment build-up, which can reduce the reservoir's water storage capacity and impact turbine efficiency. Addressing this requires periodic dredging. Turbines and generators also face wear and tear and need regular inspections, maintenance, and potential replacement. Concrete structures might develop cracks due to age and environmental factors, necessitating repair to ensure structural integrity. Lastly, spillway gates and other mechanical systems require regular checks to ensure they function correctly during flood events and standard operations.

Small-scale hydroelectric projects, often called micro-hydro or mini-hydro, can be surprisingly efficient in specific contexts. While they generate less absolute power than large-scale dams, their efficiency is based on a different metric: the ratio of energy produced to the potential energy available. Due to technological advancements and the direct connection to local communities, these projects can have shorter transmission distances, reducing energy losses. Furthermore, since they're usually run-of-the-river systems, they can often achieve a balance between power generation and environmental preservation. However, the initial investment and specific site requirements can influence the overall cost-effectiveness compared to large-scale alternatives.

The capacity of a hydroelectric dam, often expressed in terms of megawatts (MW), is significantly influenced by two main factors: the volume of water in the reservoir and the height difference (or 'head') between the water's surface and the turbines. Dams with larger reservoirs can store more gravitational potential energy, and sites with greater heads allow for more kinetic energy conversion as water descends. Therefore, a physically larger dam in a location with a higher elevation difference typically has a greater capacity. However, engineering constraints, sedimentation rates, and environmental considerations can also impact the actual capacity realized.