The main lake in Beaumont Park was situated in the upper section at the Butternab Lane entrance.
As shown on the above 1906 Ordnance Survey map, the main section of the lake had two small rocky islands, one of which could be accessed via an ornamental stone bridge.
The lake was laid out in the summer of 1882 and was completed in time for the formal opening ceremony in October 1883. The cost of concreting its base was recorded as £160 and the work was carried out by R.L. Lowe.
In February 1884, bank agent Gilbert D. Winter made a gift of two swans to the park and they reportedly quickly settled on the lake. A further gift of two swans was made by the Central Wards Committee in March 1887. In December 1891, one of the final acts of the Beaumont Park Committee was to authorise the relocation of two swans from Greenhead Park and the sale of one of the existing swans.
Councillor J.B. Matthewman made a gift of four wild ducks to the park in May 1887.
In April 1892, the park superintendent was given permission to empty the lake ("by discharging the water onto the playground") in order for the bottom of the lake to be cleaned.
On Friday 19 October 1894, Henry F. Beaumont was granted the freedom of the borough. During the ceremony held at the Town Hall, the Mayor presented Beaumont with a casket which had four enamel-painted scenes, that included the park's "lake and bridge", the fountain pool and the Castle Refreshment Rooms.
The winter of 1894/95 was sufficiently cold that the Leeds Mercury included the lakes at Beaumont Park and Greenhead Park on its list of places to go ice skating.
The state of the water in the lake caused concern in August 1899 and was raised by Councillor Beaumont at a council meeting. Alderman Hellawell responded that the park lakes were not emptied as frequently as he would preferred and that he would investigate the issue.
In April 1914, the Town Council considered a proposal from Councillor Boothroyd that the lake be used for open-aired bathing. The lake had just been cleaned and repaired, but Alderman Woolven felt that "the water was not sufficient for the grown-ups, but provision would be made for boys at very little expense."
On the evening of Thursday 5 August 1915, a young child fell into the lake and "was on the verge of drowning" when 12-year-old Leonard Denham of Birkby jumped in "without hesitation" and rescued the child. 
Skating took place on the frozen lake in January 1933, although concerns about lighting meant that, unlike Greenhead Park, night skating was not allowed.
The ornamental bridge appears to have been demolished at some point between 1949 and 1965, as it is absent from the 1965 Ordnance Survey map. This latter map shows that the upper section of the lake had been sectioned off and turned into a paddling pool. An iron ornamental fence, which was used to separate the two sections, still exists at this spot.
By the early 1970s, the main lake had been emptied and filled in, leaving just the padding pool. The latter was filled in in 1988.
At the south-eastern end of the lake stood a covered shelter with seating. The structure had a central supporting wall and was open on either side, with one side facing the lake and the other overlooking a grassy hill. Archive photographs show that there were small rooms situated at each end of the shelter and that they may have been heated, as the shelter's roof had two chimneys.
The construction of the pavilion was completed just prior to the park's opening ceremony on 13 October 1883.
The pavilion was eventually demolished in 1998 when Kirklees Council could no longer afford the ongoing cost of maintenance — an action which led to the formation of the Friends of Beaumont Park. 
The Friends of Beaumont Park web site contains the following recollections by Jack Merewood:
Near the pond was a large shelter which has only recently been demolished. At one end there was a door behind which gardeners implements were stored. The other end was a shop where on Sundays ice cream and sweets were sold. In various places there were slot machines where, for a penny, you could get a thin bar of Nestles chocolate.
In 2015, £93,100 of funding from the Heritage Lottery Fund was secured in order to offset some of the cost of constructing a modern pavilion near to the site of the original structure. The proposed building comprises a café with disabled toilets and an external seating area.
In 2016, it was announced that cuts to council funding meant that the plans for the pavillion were being placed on hold.
A separate lower lake was situated below the park's waterfall and was used to grow aquatic plants, specifically at the request of the Huddersfield Naturalists' Society. At its deepest section, close to the back rocks underneath the waterfall, the lake was around 3 feet deep.
In March 1892, the Huddersfield County Borough Council requested that the borough surveyor "have the rocks adjoining the aquatic lake in Beaumont Park under-pinned and rendered safe." This was apparently in relation to an earlier incident where rocks had slipped and the council had to pay £184 "in order to put matters straight."
On the late afternoon of Sunday 26 September 1886, Mary Wilson (aged 4) and her siblings Ada (10) and John (7) were standing next to the lower lake when a small boy ran past and accidentally bumped into Mary. The boy ran off, apparently without realising that Mary had then lost her balance and fallen into the water.
Unfortunately no-one else witnessed the incident and, when the distraught siblings ran to fetch help, the first adults they found assumed that Mary had fallen into the upper lake. Whilst a search was made of the wrong lake, four youths had apparently spotted Mary's body and one started to wade into the water but, for reasons unknown, he turned back and the youths walked off — the inquest into the drowning heard that these youths had not been identified and one juror felt it was a "cowardly shame" they had walked away.
Thomas Whiteley, a warehouseman of Woodfield Road, had been standing near to the Castle Refreshment Rooms when a boy ran up and told him a child had been drowned. Using a hooked pole, he waded in and was able to retrieve Mary's body.
The inquest into Mary's drowning recorded a verdict of accidental death.