Hydrology: Attributes of a Healthy Stream

An alluvial stream is one whose channel is composed of the sediments transported by the stream itself and that generally changes the shape of its bed as the rate of flow changes. (Adapted from Attributes of an alluvial river and their relation to water policy and management by Trash, McBain, and Leopold).

My Image

Attribute Four: Materials on the Stream Bed Move Periodically

As streamflows rise in the spring during peak snowmelt, often the flows move sand, gravel, cobbles and even larger rocks on the streambed. Sands move at lower flows than gravels, and gravels at lower flows than cobbles, and so on. Also, materials in some parts of the stream move before similar-sized materials in other parts of the stream. Gravels in pool tails, for example, move at lower flows than gravels in riffles.

In general streambed materials move many times a year, but in some years they may not move at all. On average, the streambed is mobilized about once a year.

The periodic mobilization of the streambed is vital to the proper functioning of a stream

  • At base flow there is little or no movement of streambed materials
  • At 30 to 40 percent of bankfull flow, the sand stored in pools begins to move downstream
  • At 75 to 85 percent of bankfull flow, the gravels in pool tails begin to move downstream
  • At bankfull flows, the gravels in riffles begin to move downstream. At higher flows, still larger materials move

Hydrology: Attributes of a Healthy Stream

An alluvial stream is one whose channel is composed of the sediments transported by the stream itself and that generally changes the shape of its bed as the rate of flow changes. (Adapted from Attributes of an alluvial river and their relation to water policy and management by Trash, McBain, and Leopold).

My Image

Attribute Four: Materials on the Stream Bed Move Periodically

As streamflows rise in the spring during peak snowmelt, often the flows move sand, gravel, cobbles and even larger rocks on the streambed. Sands move at lower flows than gravels, and gravels at lower flows than cobbles, and so on. Also, materials in some parts of the stream move before similar-sized materials in other parts of the stream. Gravels in pool tails, for example, move at lower flows than gravels in riffles.

In general streambed materials move many times a year, but in some years they may not move at all. On average, the streambed is mobilized about once a year.

The periodic mobilization of the streambed is vital to the proper functioning of a stream

  • At base flow there is little or no movement of streambed materials
  • At 30 to 40 percent of bankfull flow, the sand stored in pools begins to move downstream
  • At 75 to 85 percent of bankfull flow, the gravels in pool tails begin to move downstream
  • At bankfull flows, the gravels in riffles begin to move downstream. At higher flows, still larger materials move

Hydrology: Attributes of a Healthy Stream

An alluvial stream is one whose channel is composed of the sediments transported by the stream itself and that generally changes the shape of its bed as the rate of flow changes. (Adapted from Attributes of an alluvial river and their relation to water policy and management by Trash, McBain, and Leopold).


Attribute Four: Materials on the Stream Bed Move Periodically

As streamflows rise in the spring during peak snowmelt, often the flows move sand, gravel, cobbles and even larger rocks on the streambed. Sands move at lower flows than gravels, and gravels at lower flows than cobbles, and so on. Also, materials in some parts of the stream move before similar-sized materials in other parts of the stream. Gravels in pool tails, for example, move at lower flows than gravels in riffles.

In general streambed materials move many times a year, but in some years they may not move at all. On average, the streambed is mobilized about once a year.

The periodic mobilization of the streambed is vital to the proper functioning of a stream

  • At base flow there is little or no movement of streambed materials
  • At 30 to 40 percent of bankfull flow, the sand stored in pools begins to move downstream
  • At 75 to 85 percent of bankfull flow, the gravels in pool tails begin to move downstream
  • At bankfull flows, the gravels in riffles begin to move downstream. At higher flows, still larger materials move