In the Boulder County foothills, the creek and the hillside both carry hazard

A property in the Boulder County foothills can face two related water hazards, and a flood map only shows one of them.

The first is familiar: the creek itself, where water rises along the valley floor. The second is the hillside. In very heavy rain, steep drainages can send down a debris or mud flow — a fast, churning mix of water, soil, rock, and trees scoured off the slope. After the September 2013 storm, the Colorado Geological Survey documented flows like these in Boulder County that ran above and outside the mapped floodplain, in places people did not think of as flood-prone.

Why this matters for a buyer or owner: a parcel can be clear of the regulated floodplain and still sit below a gully or at the mouth of a steep drainage. The hazard depends on the shape of the land above the house, not only on the nearest stream line. It is worth looking uphill as well as at the water.

The Colorado Geological Survey maps debris-flow and landslide susceptibility for Boulder County, and the county keeps floodplain information current. Check both the geologic hazard maps and the local floodplain data for a specific address.