Measuring Low Flow in San Pedro River

(water flowing)

- Okay, what we're looking at here

is the gage pool at the Charleston

at San Pedro gage.

The gage pool extends downstream

to a point where

there is a change in
velocity or a flow regime.

And you can see, during
these underflow conditions,

we're looking at an area that is about

1.1 feet wide.

The deepest point of this riffle

is going to be right about...

here.

0.11 feet.

Our flume section is just
about 30 feet downstream.

We have here an area that
has been used previously

for a Parshall flume measurement,

and what I'm going to do now is

I'm gonna fill that in a little bit

with some very fine sediments.

I'm going to set it,

I'm going to fill in around the flume.

(water flowing)

This device right here is
a three-inch Parshall flume.

It's constructed in such a way that it has

a very precise throat.

Dimensions of the inside

of the Parshall flume are very precise.

It has been computed
using a rating table,

that at different elevations of water

flowing through the Parshall flume,

a discharge can be associated
with that elevation.

We actually measure the
elevation in the stilling well

of the Parshall flume.

This is the stilling well, this part here.

The purpose

is at very low flow conditions,

like we have here at the San
Pedro at Charleston gage,

we have an accurate method for determining

and computing the discharge
using the rating table.