Task: Question Answering
Input (English): Background Paragraph: Thermoclines, or sudden changes in temperature. Where the air temperature is higher than the water temperature, shallow water may be warmed by the air and the sunlight but deeper water remains cold resulting in a lowering of temperature as the diver descends. This temperature change may be concentrated over a small vertical interval, when it is called a thermocline.[3][4]
Where cold, fresh water enters a warmer sea the fresh water may float over the denser saline water, so the temperature rises as the diver descends.[3]
In lakes exposed to geothermal activity, the temperature of the deeper water may be warmer than the surface water. This will usually lead to convection currents.[3]. 
Story: Ryan explored two lakes; Big lake, and Arrow lake to the south. Arrow lake had a river flowing into it, while Big lake presented evidence of underwater geothermal activity. Many divers drowned in Arrow lake because of temperature shock. 
Question: Which lake had more of a temperature change concentrated over a small vertical interval?
Output (English): Arrow lake.