THE FEESH- WATER LOCHS OF SCOTLAND. 
21 
Loch Calder (see Plate VII.). — Loch Calder lies about 5 miles to the 
south-west of Thurso. It is a large loch, distinguished from the other 
Caithness lochs visited by the Lake Survey by its great depth. At the 
time of the survey green algse abounded in the water, and gulls and other 
birds were very numerous. The loch trends in a north-west and south- 
east direction, and is 2^ miles in length. The southern portion is narrow 
and shallow, while the northern portion is much wider and deeper, the 
maximum breadth being very nearly 1 mile, and the mean breadth of the 
entire loch exceeding half a mile. The superficial area is about 844 acres, 
or 1\ square miles, and the drainage area nearly 10 square miles. The 
maximum depth of 85 feet was observed towards the northern end and 
towards the eastern shore. The volume of water is estimated at 767 
millions of cubic feet, and the mean depth at nearly 21 feet. The loch 
was surveyed on October 6, 1902, but the elevation of the lake-surface 
above the sea could not be determined ; when levelled by the officers of 
the Ordnance Survey on May 28, 1870, the elevation was found to be 
205*2 feet above sea-level. It was stated that the water might rise 2 feet 
above, and fall about 1 J feet below, the level on the date of the survey ; 
but the level is affected by a sluice at Achavarn, which is used both by 
South Calder mill and by the Thurso waterworks. 
Loch Calder is irregular in outline, and rather peculiar in conforma- 
tion. In the wide portion of the loch, off the western shore, there is an 
island situated on a large bank surrounded by deeper water, and the 
narrow southern portion is so shallow that one must proceed three-quarters 
of a mile from the southern end before meeting with depths exceeding 
11 feet. The deep basin is contained in the eastern half of the wide 
northern portion of the loch, the deepest sounding in 85 feet having been 
taken about half a mile from the northern shore and a quarter of a mile 
from the eastern shore. Here there is a basin about a mile in length, and 
exceeding 30 feet in depth, the 50-feet basin being nearly three-quarters 
of a mile in length, and distant about a quarter of a mile from the northern 
shore. The loch, as a whole, is comparatively shallow, since 72 per cent, 
of the lake-floor is covered by less than 25 feet of water, as will be 
seen from the following table, giving the approximate areas between the 
consecutive contour-lines, and the percentages to the total area of the 
loch : — 
Feet. 
A ores. 
Per cent. 
0to25 
606 
71*8 
25 „ 50 
170 
20-1 
50 „ 75 
55 
6-6 
over 75 
13 
1*5 
844 
100-0 
Temperature Observations . — The temperature of the surface water at 
9,30 a.m. on the date of the survey was 51°*1 Fahr., while the following 
