Big Lost River Hydrology

The Big Lost River is a Western American Classic!  First, its headwaters encompass some of the wildest, tallest, rugged peaks in Idaho, if not the Intermountain West.  Second, The Big Lost never gets to meet and greet another river.  Nope, there's no confluence in The Big Lost's future--ever.  Whatever few trickles of flow can dribble past the irrigated alfalfa and grain fields of the Valley of The Big Lost will simply disappear into the lava flows of The sprawling Snake River Plain.  It's always been that way.  Water officials dub The Big Lost one of Idaho's "Sinks Drainages."   To put it bluntly, they sink outta sight.  That;s how they got their names: The Big Lost and The Little Lost.

In the above watershed mosaic, the key drainages to watch are The East Fork and Wildhorse Creek.  They contribute the lion's share of inflow to Mackay Reservoir--the all-important water storage facility for the farmland downstream.

Here's a recent hydrograph of the outflow from the upper Big Lost watershed as it prepares to pass into Mackay Reservoir.  This USGS stream gage effectively captures the all-important North East Forks as well as Wildhorse Creek.  Bear in mind, although this May 15 graph looks pretty good, it's far, far below the record flow for May 15 of 2,550 cfs logged in 1984.

This graphic shows where the USGS stream gage is located, roughly 10+ miles above Mackay Reservoir.  As of mid-May, the reservoir is over 90% full.  An incoming storm forecast for May 19-21 has potential to rain on the existing snowpack and might cause the reservoir to "fill and spill."

As far as we can determine, the last time Mackay Reservoir filled and spilled was in June 2011.

Here's an Idaho DEQ TMDL report that shows a lot of the details of The Big Lost River:

http://www.deq.idaho.gov/media/450235-_water_data_reports_surface_water_tmdls_big_lost_river_big_lost_entire.pdf