Report on Walnut Creek water quality encouraging

Raccoon River Watershed Association member participates in Polk County Conservation program

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I did chemical, physical and biological monitoring at my two sites on North Walnut Creek (NWC) July 17, in Windsor Heights and Urbandale. I have been monitoring under the IOWATER volunteer program since 2007, and this year became a volunteer in the Polk County Conservation Board (PCCB) monitoring program.

North Walnut Creek originates in the south part of Grimes and flows south-southeasterly through the western suburbs of the Des Moines metro area and enters Walnut Creek south of University Boulevard and just west of 73rd Street near west of Sam’s Club.

From there Walnut Creek flows mostly southerly and a little bit easterly to join the Raccoon River a short distance upstream of the Des Moines Water Works facility. The NWC is thus an “urban stream” as is Walnut Creek in this area. It doesn’t meander much and has relatively poor habitat for aquatic life or activities.

Both NWC and Walnut Creek are maintained extensively as greenbelt areas, however, with nice biking and pedestrian trails and natural riparian areas, and NWC has pretty good canopy cover throughout.

One of my sites is about a block north of University Boulevard, below the creek’s confluence with a large storm sewer outlet (I don’t go there when it rains.) The other site is about a block north of Hickman Road, below the confluence with a small tributary.

I used to sample at five other sites within this stretch but scaled back to these two sites this year.

Under the PCCB program, staff and volunteers sample twice a month for chemical and physical parameters, and they do a biological assessment once a year in the third week of July. This assessment tries to locate and tabulate living Benthic Macro Invertebrates (BMIs) at and upstream and downstream from the site.

BMIs are critters — mostly insect larvae and clams, snails, crawdads and worms — that live in or near the streambed. BMIs have no backbone and are visible to the eye. The existence of and types of BMIs at a given location in a stream are indicators of water quality in that vicinity.

BMIs have been categorized scientifically into three groups: pollution intolerant (high-quality group), somewhat pollution tolerant (middle-quality group) and pollution tolerant (low-quality group).

If no BMIs or mostly pollution-tolerant species are found, that is an indication of poor water quality. If many BMIs are found and many are in the pollutant-intolerant group, that is an indication of good water quality. There is of course a range of poor to good water quality in between.

Usually my two sites don’t have much going for them as far as BMIs. NWC is mostly shallow sand bottom, with habitat that support BMIs being few and far between. BMIs like partially submerged rocks in or near riffles, partially submerged logs and logjams, overhanging vegetation along the stream banks and similarly protected areas.

I had a good experience yesterday at the storm sewer site. I picked up a fairly good-sized rock at the end of a downstream riffle, and there were seven caddisfly larvae on it, four of them being in the shelters they sometimes build, both leaf and gravel.

Caddisflies are in the high quality group. This was the first time I’ve seen that many at either of these sites. There was also a small leech (low quality) on that rock, and in the rest of my investigation at this site I found one more caddisfly, two crawdads and a water scorpion (middle quality).

This was still not a large number of critters but given the larger number of high-quality species, it would get a pretty high water-quality rating for this day.

At the other site I found one small mayfly larva (high quality), and it had a small bloodworm (low quality) clinging onto one of its tails, two damselfly larvae and many (about 50) water striders (middle group) and a pouch snail (low quality).

With the predominance of the middle-quality group, one high quality and two low quality, it would get a slightly lower than average rating for this day. But there were not many critters besides the water striders.

As far as chemical quality, the main parameter of interest is of course nitrate. Nitrate levels have been zero at these sites in July, 2 milligrams per liter in June, and 5 milligrams per liter in May. NWC has somewhat higher levels of chloride than most Iowa streams, but it is not excessive by any standards that I am aware of. Other measurements have been pretty normal.

Michael Murphy is the secretary-treasurer of the Raccoon River Watershed Association. He lives in Windsor Heights.

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