Water & Watersheds: Fine Sediment in Pools
About this Research:
Notes and comments on measuring V*:
- A pool is outlined by its residual water surface, so backwaters
with connections deeper than the riffle crest depth should be included
in the pool.
- Areas under undercut banks, logs, logjams & etc are also part
of the pool. The easiest way to measure these places is often with
a short stick.
- If there are logs, wood, or rock outcrops occupying part
of the water column at your measurement spot, the part of those objects
that is in the residual pool should be subtracted from the water depth.
- if the obstructions are entirely below riffle crest depth,
measure actual water depth and subtract the height of the object.
- if the obstruction extends from the surface to below riffle crest
depth, measure the actual water depth in the water column and add riffle
- if the obstruction is entirely above riffle crest depth, measure
water depth to the pool water surface.
- Watch for soft saturated bank material near pool edges. This is
most often a problem when there are fluvially deposited sediments near
the bank. Dig up a sample and feel with your rod as you push through,
and only record the depth of fluvially deposited fine sediment.
- If a fines deposit includes a high percent of organic matter (leaves,
sticks, etc), estimate the percent of the volume that's organics and record the depth
as that percent of the actual depth (ie you measured 0.22 m of fines but it's
half organics, record fines depth of 0.11 m).
- If there are fines deposits at the riffle crest, measure the riffle crest depth as though
those fines weren't there. Probe gently with your rod to find
the shallowest continuous line across the downstream end of the pool that's made up of course
material and use the depths to that coarse material to calculate riffle crest depth.
- Never list a water+fines depth that's less than riffle crest
depth-it will make errors in the calculations. If water + fines at a location is shallower
than riffle crest it's out of the pool and you shouldn't be measuring it.
- If you go in and then out of the pool across a transect (rocks are
the usual reason) you need to have a point exactly equal to the riffle crest depth at
the point where you leave the residual pool and again when you reenter the residual pool
at the other side of the rock.