The undulating terrain (varying from about 700 m to about 962 m AMSL) in the region aided in the formation of interconnected lakes in the region (Figure 1.7).  Bangalore City was once aptly known as ‘city of lakes’ due to the presence of large number of lake (about 285 lakes in an area of 161 sq km, Spatial extent of Bangalore in 1980’s). These lakes were all interconnected with canals / drains (kaluveys’s) to enable transferring excess water to the next lake. These lakes catered the basic needs such as maintaining and recharging ground water, drinking water to the surrounding people, habitat for fishes and other aquatic ecosystems, sustaining food (fish, etc.) and agricultural activities, etc.

The drainage network in Bangalore carries water to the River Cauvery through its tributaries Arkavathi, Pinakini or Pennar and Shimsha. The central, northern and eastern portion is undulating with the upland tracts occupied by scrubs, while the low lands occupied by series of tanks formed by embanking the streams along the valley. These valleys consists of varying size water bodies from small ponds to large lakes. The southern portion of the land consists of hills that are close together and are surrounded by thick jungles.

Bangalore being located on the ridge, forms three watersheds as precipitation flows as runoff in three directions along  the valleys (Figure 1.7) - Koramangala Challaghatta Valley (K&C Valley), Hebbal Valley (H Valley) and the Vrishabhavati Valley (V Valley). Under the administrative boundary of Bruhat Bengaluru, K&C valley is the largest encompassing an area of 255 square kilometers, followed by Hebbal valley with an area of 207 square kilometers and Vrishabhavati valley with an area of 165 square kilometers. Both K&C valley and Hebbal valley joins at Nagondanahalli village (BBMP Ward 94 – Hagadur) which further flow to Dakshina Pinakini River, where as Vrishabhavati valley joins Arkavathi river which is a tributary of river Cauvery.

Figure 1.7: River and Lake network along the Major valleys

During 1800, there were 1452 water bodies with the storage capacity of 35 TMC (in the current spatial extent of 741 sq.km.). The number of lakes in Bangalore has reduced from nearly 285 (spatial extent of Bangalore: 161 sq.km. in early seventies) to 194 (spatial extent of Bangalore: 741 sq.km. in 2006). Unplanned rapid urbanisation during late nineties, witnessed large-scale unrealistic, uncontrolled developmental activities in the neighborhood of lakes, which  led to

1. encroachment of lakes and storm water drains resulting in decline in ground water table, while increasing the instances of flooding;
2. dumping of solid waste (MSW), Construction debris, etc. in storm water drains, lake catchment and in lakes.;
3. sustained inflow of partially or untreated sewage, polluting existing surface and subsurface water resources;
4. reduced water holding capacity due to accumulation of silt; construction debris, etc.;
5. topography alterations in the lake catchment; and
6. sustained inflow of untreated industrial effluents.

During the last four decades there has been 79% reduction in water bodies and the number of lakes in Bangalore is given in Figure 1.8.1. Loss of interconnectivity is evident in Figure 1.8.2 due to encroachments.