Select Page

Retention vs Detention Storage

When drought-weary observers of winter flood flows yearn for more storage capacity they are usually thinking of reservoirs retained by dams.  But opportunities for detention storage are off most folks’ radar.

That needs to change, especially since the opportunities to expand/ restore detention storage are vast, in California and doubtless elsewhere.  Humans have been unwittingly degrading watershed detention functions since our mastery of fire.

As defined in this site’s Glossary:

Retention: water is held against the force of gravity, above or within the soil.  Within the soil, retention occurs within capillary pore spaces (micropores), where the retained water is available for uptake by plants.  Above-ground retention is accomplished by dams or similar obstructions.

Click image to expand

Detention: temporary storage of water.  Below ground, soil water drainage by gravity is slowed, though not stopped, in macropores.  In fact, macropores are among the primary conduits for “preferential flow” through the soil profile.  Above ground, runoff is typically detained by snow, but may be slowed by other semipermeable means — most typically by routing through vegetation and soil, but other means are possible.  Beavers are engineers par excellence of detention storage.

Restoration of detention storage to lands with degraded infiltration and percolation functions is far less costly to establish and maintain than engineered retention storage .  

Watersheds/ catchments restored with native cover types – uplands, along with expanded riparian zones, meadows and other floodplains – will become increasingly self-sustaining, while their detention storage capacity increases over time.  What’s not to love about that?