Talk:Columbia Slough

Minimum flow

From the article:

and the minimum flow was -6,700 cubic feet per second (−190 m3/s) on February 7, 1996.[4] Flows can reverse direction during the tidal cycle.[4] These reverse flows consist of fresh water rather than brackish water because saltwater intrusion up the Columbia River stops at about 23 miles (37 km) from the ocean.

Not an expert but this seemed like an unlikely explanation for me, given how far this river is from the ocean and how small it is (with average flows over 60 times smaller than this figure), so I checked the reference (4):

minimum daily discharge, -6,700 ft³/s Feb. 7, 1996

So that looks good, but still seemed odd, so I searched google for that date and found flood warnings on the Columbia river that day, so it seems much more likely the reverse flow was caused by this, rather than tidal effects. --81.153.144.130 (talk) 13:56, 23 February 2010 (UTC)

Perhaps high water due to flood increased the overall volume of water and tidal effects caused the reverse flow? I would guess that in general the reverse flows are due to tidal effects--which occur as far upriver as Bonneville Dam. That the largest reverse flow coincided with flood stage high waters is perhaps why the volume was so high? Not sure where to look for a source on the topic. Pfly (talk) 16:44, 23 February 2010 (UTC)

Good question. I, too, wondered about the big negative number but found nothing to fully explain it. The flood hypothesis seems plausible and the flood plus tidal effects even more so. During the monsoon season, the Mekong River causes a hefty reverse flow on the Tônlé Sab River, according to a source here. Now if only we can find a reliable source that gives the same explanation for the slough... Meanwhile, I could add a note explaining that the high reading occurred on the day of a flood warning on the Columbia. Would that be a good idea? Finetooth (talk) 17:20, 23 February 2010 (UTC)

Such a note would be good—especially if it linked to a description of the widespread occurrences of that time. The U of O link has a narrow (dimensionally speaking) view of it. —EncMstr (talk) 18:05, 23 February 2010 (UTC)

Agree a note or some mention would be good. The maximum flow rates measured on Larrys Creek were during Hurricane Agnes and Hurricane Ivan (two different gages) which is mentioned in that article. Ruhrfisch _><>°^° 19:00, 23 February 2010 (UTC)

Added a sentence to the "Discharge" subsection that links the flood to the maximum reverse flow. I used the U of O link, but I'm thinking of added a "Notes" subsection at the bottom that could include a more detailed note of the sort suggested by EncMstr. Thanks to 81.153.144.130 for spotting the problem and finding the U of O source. Thanks to all for help with the spate of vandalism. Finetooth (talk) 20:18, 23 February 2010 (UTC)

Hmm, after reading again and thinking I'm not sure about how it reads now, at least reference-wise. My thoughts:

• The maximum flow was 2,400 cubic feet per second (68 m3/s) on December 5, 1995, and the minimum flow was −6,700 cubic feet per second (−190 m3/s) on February 7, 1996. This is referenced to the USGS gage on the Columbia Slough and is of course fine.

• Flooding can cause reverse flows on river tributaries; the minimum flow (biggest reverse flow) on the slough coincided with a flood on the Willamette. This is referenced to that flood warning page, here. The reference backs up the second part of the sentence but not the first--the minimum flow (max reverse) happened during a flood, yes, but the page does not say that reverse flows can be caused by flooding. Also, the phrase ...on river tributaries confused me--which river tributaries? I think I just got it while typing now--if a river floods the water can flow up into its tributaries? Anyway, the reference doesn't back this up.

• Flows can also reverse direction during the tidal cycle. Referenced to the USGS gage page. All fine.

• The tidal flows consist of fresh water rather than brackish water because saltwater intrusion up the Columbia River stops at about 23 miles (37 km) from the ocean. Referenced to this USGS CVO page. All fine--although the wording seems slightly confusing to me. I could try to rephrase it, but I almost think the original wording may be better:

and the minimum flow was -6,700 cubic feet per second (−190 m3/s) on February 7, 1996. [USGS gage ref] Flows can reverse direction during the tidal cycle.[USGS gage ref] These reverse flows consist of fresh water rather than brackish water because saltwater intrusion up the Columbia River stops at about 23 miles (37 km) from the ocean. [USGS CVO ref]

This seems like a logical sequence of three things: 1) The minimum flow was negative, which looks weird and needs explanation. 2) The negative number means the flow was backwards/reverse, and the very page that the flow data comes from notes that tidal effects can cause reverse flows. 3) The terms "tide" and "tidal" suggest salt water, but the Columbia Slough location on the Columbia is above the reach of salt water.

Perhaps it would help to explain how the Columbia Slough, while above the reach of salt water, is still well within the reach of tidal effects. The USGS CVO page says, "Salt water intrusion into the Columbia River estuary reaches about 23 miles upstream from the mouth. The effects of tides upon the flow rate and level of the river are felt up to Bonneville Dam, river mile 146.1." And the fact that the minimum flow (max reverse) coincided with a flood should perhaps be mentioned without explicitly claiming how the flood affected the slough's flow--at least pending finding more sources.

Sorry for being so wordy. As usual I wrote more than I meant to. Curious topic. I can try to edit the page to address the points above, but I might try to find info about floods and reverse flows first. Pfly (talk) 21:14, 23 February 2010 (UTC)

Thanks, Pfly. Please be bold and revise. This section has been a problem for quite a while, and it sounds as if you have a handle on it. Finetooth (talk) 21:43, 23 February 2010 (UTC)

Hehe, okay. I'm looking into it, but nothing much yet. I did discover we have a page about the flood in question: Willamette Valley Flood of 1996. Pfly (talk) 22:20, 23 February 2010 (UTC)

I couldn't wrap my head around this yesterday but came back to it today, installing a notes system that I hope improves the prose flow of the main text by moving complications to the notes section. Although I haven't found a source that says that the occasional flood overwhelms the importance of the diurnal shifting of the tides, I think most readers will infer that from the details in the notes and the description and image of the Vanport flood. Anyway, I hope things are now more clear and reasonably sourced. Finetooth (talk) 19:55, 24 February 2010 (UTC)

Looks great. I found a source or two about flood waters backing up into tributaries in floodplains, but specific to the Mississippi River or elsewhere. Also found a source or two about the tides causing reverse flow most strongly when water levels are low. Trying to piece it together got complicated and I had to give up for the night. But although I didn't find anything about the Columbia Slough specifically, it seemed clear that the min (max reverse) flow was almost certainly caused by the flood, not the tide. It was a big flood--the Willamette at Portland crested at 28.6 ft--the "flood stage" is 18 feet and the record high is 33 feet (during the huge flood of 1894). The Columbia Slough's reverse flow during the flood was several times larger than its peak "regular flow" ever. The Willamette must have simply spilled out into the slough, and all over its floodplain, in large amounts. Anyway, nice job, looks great. Think a link to Willamette Valley Flood of 1996 would be useful? Pfly (talk) 21:12, 24 February 2010 (UTC)

Agree it looks great - also think a link to the flood article (could just be piped to the current word flood) would be useufl. Ruhrfisch _><>°^° 21:20, 24 February 2010 (UTC)

Glad you both approve. Yes, the link to the flood is a good idea; I have now added it. Finetooth (talk) 22:31, 24 February 2010 (UTC)

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