Exceedence Flows

"Exceedence" is a way to describe the percentage of time for which an observed stream-flow is greater than or equal to a defined stream-flow. Exceedence is used when stream-flow data are not normally distributed (i.e. on a bell-shaped curve). Most streams flows are not normally distributed because high flow events can skew the data making the mean flow greater than the median flow.


Low-flow events have high exceedence percentages, and high-flow events have low exceedence percentages. Low-flow events have a high exceedence percentage because most of the time, observed flows exceed the low flow. Similarly, high-flow events have low exceedence percentages because most observed flows are lower than the high-flow levels.


A graph showing the relationship between flow and exceedence. A low flow has a high exceedence because most of the time the observed flow will exceed the low flow. A high flow has a low exceedence becaues an observed flow exceeds a high flow level only rarely.

Figure 1. The mean monthly flow for the month of March for a hypothetical stream.


In the example illustrated by the Figure 1, a hypothetical stream has a mean monthly flow in March of 45 cfs, and a median monthly flow of 38 cfs. The 50% exceedence flow in March is the median flow of 38 cfs. This means that the median March flow 5 out of every 10 years will be greater than or equal to 38 cfs.


The stream has an 80% exceedence flow of 25 cfs. An 80% exceedence flow means that the median March flow will be equal to or greater than 25 cfs 8 years out of 10.


The stream has an 10% exceedence flow of 75 cfs. An 10% exceedence flow means that the median March flow will be equal to or greater than 75 cfs 1 year out of 10.


The algorithms proposed for establishing instream flows, future allocations, and the floor flow are based on exceedence values.


The individual instream flows will be decreed as quantities in cubic feet per second (cfs) as will the future allocation for non-DCMI (non- domestic, commercial, municipal, and industrial) uses and floor flows. The administrative provisions for these in stream flows will, however, recognize they are being established based upon estimated flow unless measured flows are available or acquired. The provisions of the final decrees will provide a mechanism for changes to these decreed amounts based upon actual flows if such data become available.


To prevent dewatering streams by future non-DCMI use, future non-DCMI use would be curtailed at a floor equivalent to the unimpaired monthly 80% exceedence flow. Consequently, the flow values for the four categories will be as follows:


  1. For State and private basins
    instream flows would be decreed for each month of the year at the 50% exceedence level of the estimated unimpaired flow, subordinated to a future non-DCMI use in the amount of 25% of the lowest median monthly unimpaired flow value.
  2. For federal, non-wilderness basins
    instream flows would be decreed for each month of the year at the 40% exceedence level of the estimated unimpaired flow, subordinated to a future non-DCMI use in the amount of 10% of the lowest median monthly unimpaired flow value.
  3. For federal wilderness and Wild and Scenic basins
    instream flows would be decreed for each month of the year at the 30% exceedence level of the estimated unimpaired flow, subordinated to a future non-DCMI use in the amount of 5% of the lowest median monthly unimpaired flow value.
  4. The Special Areas
    include watersheds that hold special values including high value habitat for fish resources, other special values, and areas where future development opportunities would be preserved. The instream flows and reservations for future non-DCMI use for the special areas differ from the land based formula described above.

Special Areas include:

Lower Salmon River below Long Tom Bar to the mouth:

Instream flows for the lower Salmon River downstream of the Wild and Scenic Reach would be consistent with the application filed for the lower Salmon River below Hammer Creek. The State application for the instream flow in the Lower Salmon addresses the reach from the mouth to Hammer Creek. The instream flows reach in the current application will be extended to include the reach of the Salmon below the Little Salmon. The instream flows in the reach between the Little Salmon and the Wild and Scenic River will be based on the downstream reach and adjusted for the inflow from the Little Salmon River. The State instream flow will be made consistent with the Wild and Scenic instream flow for the main Salmon River.


South Fork Salmon River and tributaries contained within the Tribal Priority Stream List:

Instream flows would be decreed for each month of the year at the 40% exceedence level of the estimated unimpaired hydrology, subordinated to a future non-DCMI use in the amount of 5% of the lowest median monthly unimpaired flow value.


Upper Salmon

The upper Salmon basin includes a number of tributaries that meet the criteria of "B" List streams. Instream flows established for the tributaries or the mainstem Salmon will be in accord with Wild and Scenic River instream flows and future allocations, subject to the Order Approving Stipulation and Dismissing Objections in Consolidated Subcase Nos: 63-25239, 75-13316, and 75-13606, issued by Judge Daniel C. Hurlbutt, Jr., Presiding Judge, Snake River Basin Adjudication, on June 16, 1998.


Lolo Creek

Instream flows will be decreed for each month of the year at the 40% exceedence level of the estimated unimpaired hydrology, subordinated to a future non-DCMI use in the amount of 10% of the lowest median monthly unimpaired flow value.


Bedrock Creek

Instream flows will be decreed for each month of the year at the 40% exceedence level of the estimated unimpaired hydrology, subordinated to a future non-DCMI use in the amount of 10% of the lowest median monthly unimpaired flow value.


Upper North Fork Clearwater River, Breakfast Creek

Instream flows would be decreed for each month of the year at the 40% exceedence level of the estimated unimpaired hydrology, subordinated to a future non-DCMI use in the amount of 10% of the lowest median monthly unimpaired flow value.


Future Uses for "A" List Streams

The future use allocations will provide water for non-DCMI uses. The parties will study the overlap of existing uses and future use to determine if additional criteria will assist the parties in allocating future use. The goal is to avoid reducing streamflows to a level where the unimpaired 80% exceedence value is the flow that the normally occurs in the stream due to the combination of existing and future use.