The defeat of Allen’s forces has been termed the battle of Longue-Pointe, but there is some uncertainty as to exactly where that battle took place. In this post I discuss where I believe the battle occurred. The battlefield area is shown in the map below. Montreal is at left (southwest) and the modern city of Longue-Pointe is at right (northeast). Both communities are on the Island of Montreal, and on the north shore of the St. Lawrence river.
The map shows several places referred to in this post. A - Where the citizens of Montreal assembled before the battle, B - Ruisseau Migeon (English: Migeon Creek), about 2 1/3 miles from Montreal (in 1775), C - Original site of the “Pierre-Joseph-Picard” house or “Maison Allen," which, according to tradition, is where Ethan Allen was captured, D - Ruisseau des Soeurs (English: Nun's Creek), later called Molson’s Creek, about 3 1/2 miles from Montreal.
The site of the battle was more-or-less forgotten during the 19th Century. Benson John Lossing went looking for the battlefield in the mid-19th Century. He met in Montreal, “An intelligent gentleman, who was one of the leaders in the rebellion there in 1837, assured me that the spot was unknown to the inhabitants, for tradition has but little interest in keeping its finger upon the locality, and not a man was living who had personal knowledge of the event. It is probable that the northern suburbs of the city now cover the locality, and that the place is not far from the present Longueuil ferry-landing.” 
In 1875, the Numismatic and Antiquarian Society of Montreal arrived at a similar conclusion:
“The locality of Allen’s landing and the battle ground is unknown, but it is probable that the suburbs of the city now covers it, and that the place is not far from the present ferry-landing at Hochelaga, on the road to Longue Pointe.” 
Since then, Gustave Lanctôt (1967) asserted that the battle was fought at Ruisseau des Soeurs (see D, on the above map), a creek that fed into the St. Lawrence near what is today the western edge of the city of Longue-Pointe.  Lanctôt did not cite a source for this information, which leaves the basis for the attribution unclear (at least to me). However, this stream stood near the “Pierre-Joseph-Picard” house, also known as “Maison Allen” that stood near this creek (see C, on the above map). Allegedly, it was here that Allen was captured.  Louise-Amelie Panet-Berczy painted the nearby stream in 1839 and termed it the site of the “Combat de la Grange” of 25 Septembre 1775. 
However, there are other sources of evidence about the site of the battle, and these provide reason to believe that the Hochelaga location suggested by Lossing and the Montreal Antiquarians (see B, on the above map) may in fact have been the true site of the battle.
The other sources of evidence can be found in 18th Century correspondence and memoirs. These sources indicate:
- The battle was fought between 2 and 3 miles of Montreal. 
The mean distance among the various estimates seems to be a point between Ruisseau Migeon and Ruisseau des Soeurs. The former, however, falls within the range of estimated distances, while the latter does not.
- The battle was fought “near Colonel Christie' s farm.” 
Christie was an unusual name for this area. Finding 18th Century land ownership records for this area would go a long way towards definitely establishing the site of the battle. However, I have not conducted such a search.
- The battle was fought at “the little river Truteau.” 
Some streams in this area were known by more than one name. To the best of my knowledge, none of these streams is known to have been called Ruisseau Truteau. This statement indicates that the battle was fought by a stream, but it does not indicate which one.
- A prominent ditch was perpendicular to Allen’s position. 
Allen indicated that his force was arrayed behind trees and buildings. When the British started to turn his right flank, Allen dispatched nearly half his force to a ditch that protected that flank. Ruisseau Migeon makes a 90-degree bend and then flows in an almost straight line parallel to St. Lawrence. It is impossible to say what conditions were like on the ground in 1775, but plausibly this stretch of stream was both ditch-like in appearance and suitable for the defense. Small streams on the island are known to have sometimes run dry , and September, 1775, appears to have been a time when some small streams ran dry. 
- Allen ran 1 mile away from the battlefield before he was captured. 
The “Maison Allen” (see C on the map above), which is associated with Allen’s capture was 1 mile from Ruisseau Migeon, and in the direction Allen was likely to have fled.
Ruisseau Migeon is located at about the spot Allen likely landed when crossing the St. Lawrence from Longueuil. Thus, Allen, who in some respects was unfamiliar with the area, should at least have known about this stream. Allen allegedly advanced towards the Quebec suburb of Montreal in the hopes of sparking a mass uprising, but then fell back.  Ruisseau Migeon would have been the first good defensive site he would have come across and it (presumably) had the advantage of being near where his boats were moored.
There are strong indications that Ruisseau Migeon was the site of Ethan Allen’s defeat on September 25, 1775. A definite attribution cannot be made, however, without land ownership records and/or better information about the other names for “river Truteau.”
The Site Today:
Combining 19th Century maps of Ruisseau Migeon with the details in Allen’s memoir suggests that the British and American forces might have been arrayed during the battle of Longue-Pointe as follows.
Street maps of Montreal dating from the late 19th Century to the early 20th Century show the course of Ruisseau Migeon before it was obliterated by urban development. Using the Google Maps Street View feature, one can take a virtual tour of the area today. See images below.
Google Maps’ street view that may show a part of the battlefield (click to enlarge). The British advanced from Montreal (note skyscrapers in distance) along a road that has since become the busy highway pictured. The St. Lawrence is behind the buildings at left. Ruisseau Migeon flowed from right to left and crossed the road at a point roughly between the two approaching cars closest to the viewer. If this was the site of the battle, then Young’s 10-man detachment was perhaps arrayed in the parking lot shown at left (in 1775 this was behind the creek). Allen’s force would have been arrayed behind trees and buildings that stood to the right (including beyond the right margin of this image). Young’s force was defeated with little difficulty; Allen was forced to flee after his right flank was turned (this area is not shown).
2. Benson John Lossing (1851). The pictoral field-book of the Revolution...
3. Book link.
4. Gustave Lanctôt (1967). Canada & the American Revolution, 1774-1783.
5. See this March 16, 2008 article in the Montreal Gazette.
6. See this 2008 issue of the Bulletin de l'Altier d'histoire de la Longue-Pointe.
7. A number of primary sources appeared in a previous post. Estimates were: 1 league or less (a league was about 2.5 miles; Livingston to Montgomery), about 1 league (Sanguinet memoir), within 3 miles or less (Quebec Gazette, September 28, 1775; Johnson to Legge), about 3 miles (Carleton to Legge).
8. See this letter dated October 1, 1775.
9. Letters to Quebec Gazette of September 28 and October 19, 1775.
10. From Ethan Allen’s memoir.
11. A point of discussion on the website Under Montreal.
12. See this letter from James Livingston to Philip Schuyler, written in September, 1775.
13. For links to 18th, 19th, and 20th Century maps of Montreal, see here.
14. From Ethan Allen’s memoir.
15. From Simon Sanguinet's memoir.