THE FRESH- 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 algae 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 1J 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. .Acres. Percent. 



Oto25 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 C> 1 Fahr., while the following 



