Lough Derg, the lower of the two larger lakes on the River Shannon, is located within the limit of the last glaciation and in an area of active ice-erosion. Evidence of its origin by ice erosion is seen through the irregularity of the lake floor and by the relationship of this irregularity to the various rocks (1). The southern portion of the lake was formed by ice being forced through a narrow valley between hills.

The lake lies at the southern limit of the central plain. It is bordered by Co. Glare on the south, by Co. Galway on the north and west and by Co. Tipperary (North Riding) on the east. It lies at 33.5 m above sea level and has a surface area of 117.50 km2. Lough Derg extends from Portumna in the north to Killaloe in the south with a maximum distance along its north-south axis of 35 km. Its maximum width is 14.5 km across the Scarriff-Youghal Bay transect, but the average width is less than 5 km.

The extreme northern portion of the lake, encompassing Portumna, Slevoir and Cloondavaun Bays, is relatively shallow with a mean depth of 4 m. The shores in this area are flat and there are extensive reed beds in the bays. An axial trench descending to 29 m occupies the central portion of the lake (1). The Scarriff to Youghal Bay section of the lake is uneven in depth, and a small trench fringing Parkers Point descends to 36 m. The long, narrow, southern reach of the lake extending to Killaloe, has the greatest water abstraction for domestic and other uses are presently made at Portumna at the northern end of the lake and from the lake outflow for the city of Limerick (pop. -70,000). A further abstraction is being developed for a rural water supply scheme for the Co. Tipperary (NR) area to the east of the lake.

Lough Derg is unique amongst Irish lakes in that it was the subject of biological research in the early years of this century. The reports of this work form a valuable background in comparison with the findings of recent surveys and indicate that a significant increase in the abundance of planktonic algae had occurred in the lake between 1922 and the period 1976- 1978 (4). Further increases in the levels of algal development have been noted in recent years giving rise to concern amongst angling and tourism interests along the lake shore and also at the water abstraction plant for Limerick City on the River Shannon downstream of the lake.