How Does Sites Reservoir Actually Work? The Engineering Explained

A 1.5-million-acre-foot reservoir sounds impressive — but how does it actually function? Understanding the engineering helps you appreciate why this project matters so much, and what it means for the communities surrounding it.

Off-Stream — and That Changes Everything

Traditional reservoirs dam a river, flood a valley, and disrupt fish migration in the process. Sites Reservoir works differently. It's an off-stream reservoir, meaning no river is blocked and no valley is flooded.

During major storms, when the Sacramento River runs high, water is diverted at two existing points — Red Bluff and Hamilton City — and moved through existing canal infrastructure to the reservoir site about 14 miles away in the grasslands west of Maxwell. When stored water is needed during drought or a dry summer, it's released through canals and a new pipeline near Dunnigan, re-entering the Sacramento River system.

No fish migration is disrupted. In fact, Sites Reservoir will actively benefit salmon by releasing cooler stored water to maintain healthier river temperatures during critical periods.

What Gets Built

The Two Construction Packages

Why the Sacramento Valley?

The region's geography is uniquely suited to this purpose. The broad valley floor is flanked by rolling foothill terrain that creates natural bowl-like topography — ideal for off-stream storage without requiring massive additional earthworks. Existing canal infrastructure means water can be moved efficiently without building entirely new systems from the ground up.

This location has been considered for a reservoir for over 70 years. The science and geography have always made sense. What changed is the urgency — California's climate is shifting, and flexible storage has become a necessity.

Workers need places to live. Contractors need staging areas and office space. Businesses need to support a workforce that will number in the thousands at peak construction. The commercial real estate that supports all of this becomes critically important — and scarce — fast.

Next up: Post 3 explores what may be Sites Reservoir's most remarkable feature — its unprecedented commitment to environmental water storage.