Sunday, August 8, 2010

Tanque Verde Creek

Kendall Kroesen
Tanque Verde Creek has been in the news lately so Saturday morning I set out to have a look.

First I stopped at Tanque Verde Loop Road. The broad reach of the recent flooding was obvious, especially in the corrals to the southwest. I headed east and turned into the Forty-niners golf club community. I saw a lot of cleanup activities along the streets closest to the wash.

Then I stopped at Wentworth Road. The dirt road crossing the wash was closed since the county hasn't bladed it smooth yet, so I parked up the street and walked upstream.

At first the wash was fairly wide and had patches of cottonwoods, willows, elderberries and ashes. Between these patches I saw dense mesquite forest.

It wasn't particularly early in the morning but some birds caught my attention. Cardinals sang from unseen perches and several phainopeplas whistled as the moved from treetop to treetop.

As I got a bit farther along, I found the creek was running. The water wasn't running down all the way from the mountains, but instead was draining into lower areas of the creek bed out of saturated sand and silt. Two children and their mother were catching pollywogs with little nets. Four dogs, ranging from a large, cream-colored lab to an absurdly small chihuahua, emerged from a nearby house. It was a barking charge at first but by the time they got halfway too us they were silent and wagging tails.

The house they came from look fine even though it was immediately adjacent to the lowest part of the wash--I couldn't tell for sure whether floodwaters had entered under doors or around window, but nothing looked very amiss from the outside. A bullfrog bellowed from pond by the house, which appeared to have been muddied a bit by the flood.

A little farther up a red-tailed hawk cried from near the top of a cottonwood. Later from another angle I saw a nest up there. And a little farther up a Harris's hawk croaked from a tall mesquite.

At this point—maybe a half-mile up--the wash bed became wider, and there were fewer big trees. I suddenly felt alone in a very big place. There were few structures visible from the wash, and I seemed to have emerged into another era. A Cassin's kingbird swooped nearby. I was brought back to now when I found parts of what I took to be crayfish--a non-native interloper--that had not survived the flood.

There was little evidence of fast-moving floodwaters here. The big, wide floodplain had opened its arms and accommodated the water.

This is true of much of Tanque Verde Creek. If you are in the creek bed when the water comes, of course you are in grave danger. But other than that it is only where people have built structures on the broad floodplain does any damage occur. And relatively little is swept away or damaged here by the velocity of the water, since it is relatively wide and gentle in the upper part of the floodplain. The damage comes from muddy waters rising and entering homes.

Already there have been some renewed calls for dikes that would keep water away from homes, particularly in the Forty-niners area. But there have been at least equally strident calls from local residents to do nothing. Some residents know that the safety of dikes and soil cement comes with a big trade-off.

As soon as you constrain floods into narrower passages, the water runs deeper. As water runs deeper, it runs with more velocity and power. More velocity means it scours out the sand in the wash. You are left with a deep, sandy channel--something like the Rillito or Santa Cruz River. The latter are dead hulks of rivers. They hold floodwaters in a small area, but reduce the “sponge effect” of the big floodplain. The sponge soaks up broad, slow-moving floods, recharging the aquifer and keeping forests alive.

When streams are channelized the living, rural floodplain becomes the "historic floodplain." High, dry and dead.

I walked back downstream past a flock of lesser goldfinches drinking from the stream, and frightened up a blue grosbeak. Two tropical kingbirds flitted ahead of me. There are some problems with this wash that we can fix--like invasive plants. But once we take the floods off the floodplain, we will lose everything.

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