English: Lahar (hyperconcentrated flow deposit) in the Holocene of Oregon, USA.

"Lahars" are simplistically defined as volcanic mudflows - volcanic ash mixed with water (derived from melted snow or rain). In reality, lahars are not restricted to fine-grained sediments suspended in water. Lahars are flood events that can include relatively little gravel or an abundance of gravel - sometimes very coarse-grained.

A lahar can be a debris flow, with about two-thirds sediments and one-third water. Debris flows are so sediment-concentrated that boulders are suspended and stay at the top when the flow stops - debris flows have large rocks at the surface. This type of lahar is like wet concrete in consistency - it has a high viscosity. Once moving, such flows become sheared and end up moving quickly, more quickly than water.

A lahar can be more dilute than debris flows. Hyperconcentrated flows have abundant fine-grained sediments held in suspension by turbulence. The sediment volume of hyperconcentrated flows varies - they can be clay-rich or sand-rich, with 10% solids and 90% water to a 50-50 mix. Such flows can also have large clasts, but when actively flowing, any large rocks will appear occasionally at the surface, then disappear below. Traditionally, a flow with less than 50% gravel is called a "mudflow".

The deposit seen here is a Holocene-aged lahar in northwestern Oregon that is sourced to the Mt. Hood Volcano. The sediments are coarse-grained sand with lighter-colored, finer-grained partings of silt and fine-grained sand. The exposure dates to about 1,500 years ago, during Mt. Hood's Timberline eruptive phase. Three lahars are present at this site, ranging in age from about 300 to 600 A.D.

Locality: streamcut upstream from the Sandy River Delta, western side of the Sandy River & next to Harlow Road, south of the Columbia River & north of Interstate 84, eastern side of the Portland urban area, northwestern Oregon, USA