Had to do a little research to understand what was meant by "spate," being my American self. Dictionaries appear to suggest the term refers to the state of a river, rather than a type of river, which is how I interpret the text here, but I note your source specifically says "spate river." Anyway, in your judgment, is this term well understood by most audiences? If not, I recommend just leaving out spate in the lead as the term appears to be described a bit more under Geology.
There is another source, which I didn't use, which describes a spate river as "is a river which runs fast, wide and deep when there has been lots of rain..., but slows and shrinks to almost a trickle during periods of low rainfall." I can see how it might be confusing though so I have removed it from the lead and added a bit more clarification to the geology section.
History
Who is Felix Magennis? If more detail is available, I think revising the first sentence here to say, "Felix Magennis, a ________, built a castle next to the mouth..."
I have added clarification.
18th Century --> the C in Century should be decapitalized.
done.
Castle bridge is located near the mouth --> Should the b in "bridge" be capitalized?
fixed.
Geology
"spate" Are quotations necessary here, particularly if your source (and presumably others) do not use them?
Removed. I can't remember if I was the one to add them and if I did, why, but you're right they're not needed.
Where the gradient is shallow it is made up of a sequences of riffles and pools, and the river bed is composed of pebbles and scattered boulders, with sandy margins. Where the gradient is steep the flow is faster with rapids and cascades, and the bed is composed of bedrock and boulders. These statements are pulled directly from the source and should be rephrased.
At time I had thought I had suitably paraphrased it but looking again I can see how it might be a bit too close. I have rephrased it.
Flora
It is also the only know location --> "know" should be "known"
Fixed.
Fauna
Invertebrates species --> "Invertebrate species"
Fixed
Pollution and fish kills
Water Service carried out an investigated --> "investigation"
Fixed.
In July 2006 a pollution event resulted in the death of an estimated 1,400 fish, mostly juveniles. It was caused by discharge of sewage from a pumping station. --> This can probably be condensed into a single sentence rather than splitting them up: "In July 2006, sewage discharged from a pumping station resulted in..."
2a. it contains a list of all references (sources of information), presented in accordance with the layout style guideline.
It might be helpful to add in the URLs to the subscription-only sources available through Highbeam. It's possible (not likely) that people would want to purchase access the article there, but The Wikipedia Library offers free 1-year accounts on Highbeam to editors who might find it useful to access these sources to expand this or other articles.
Done.
2b. reliable sources are cited inline. All content that could reasonably be challenged, except for plot summaries and that which summarizes cited content elsewhere in the article, must be cited no later than the end of the paragraph (or line if the content is not in prose).
Geology
The bedrock of the river as it flows through Tollymore is shale from the Silurian age, about 440 mya... appears to be sourced to p.7 of this newsletter, which is less specific:
the bedrock (Silurian age) that underlies the park was once deep seabed with sands and silts trapped between two continents as they collided into each other more than 400 million years ago.
Was the 440 mya someplace else, perhaps? If not, the prose should reflect the source.
I can't remember where I got the 440 mya from and I can't find another source now so I have changed the article.
Flooding
This Down News source doesn't have any information on the August 2008 flooding described in the initial part of this section. Perhaps the year is just wrong, and it's supposed to read 2010 in the WP article instead of 2008?
I accidentally moved the correct source to another paragraph when I updated the section recently. It has now been fixed.
I'm having trouble finding some of the other information here in the above source:
The Rivers Agency had commissioned a flood alleviation scheme along the nearby Burren River to reduce the risk of flooding in that area to 1 in 100 in any given year (national standards). This was completed in August 2007. --> I see the article discusses the flood alleviation scheme, but other details like the national standards and completion date are unavailable here.
This is also in the DARD source which was accidentally moved. The url for it was dead and I couldn't find an archived version so I removed it, but I have a copy of the pdf on my computer and can confirm that this was the original source I used for this information and for the 2008 flooding.
Looking over the table in the Irish Naturalists' Journal source, it looks laurel shrubs of the Laurus nobilis species are also fairly common (10% of coverage), and worth mentioning in the Flora section at the end.
I have added that in.
I was able to find an archaeological publication that has some content that could be useful for the article about findings of flint weaponry around the river:
McComb, Anne (2005). Random Finds of Flint Objects in the Area of the Shimna River, near Bryansford, County Down. Ulster Journal of Archeaology, 64, 172-173.
If you have access to this, check it out and let me know what you think. If not, I can have it sent to you.
I have added a bit of info from this article to the history section. Let me know if you think it should be in a different section or if any more detail should be added.
3b. it stays focused on the topic without going into unnecessary detail (see summary style).
No concerns about article size or instances where details might be excessive.
4. Neutral: it represents viewpoints fairly and without editorial bias, giving due weight to each.
Good documentation in the Fish kills section. For instance, looking at the 2009 fish kill, information about the incident itself, the investigation, and the response from the company are all reported with appropriate weight using neutral language.
5. Stable: it does not change significantly from day to day because of an ongoing edit war or content dispute.
Article was largely inactive for two years until improvements began this month.
6.Illustrated, if possible, by media such as images, video, or audio:
All images in the article are either in the public domain, have no known copyright restrictions, or have appropriate Creative Commons licenses. Searches using Tineye revealed no issues with image sourcing.
Images match very well to content explicitly discussed in the article related to wildlife, geology, and particular areas along the river such Foley's Bridge and The Hermitage. The captions work well with their images. As a side note, I notice the lot of Category:Rivers of Northern Ireland lack maps detailing where the rivers are, and I suspect there is not a free map image for this one either, but if you are aware of one, it would be a great addition to the article.
I did look for a map but unfortunately I couldn't find an appropriately licensed one.