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Thalweg - The line defining the lowest points along the length of a river bed or valley. Also the line defining the central (long) axis of a buried channel or valley.
Thrust Faulting - A shallow dipping fault in which the hanging wall moves up relative to the footwall. It is caused by horizontal compression. This results in placing older rock over younger rock.
Till - A sediment deposited directly by a glacier that is unsorted and consisting of any grain size ranging from clay to boulders.
Total Dissolved Solids - Concentration of all substances dissolved in water (solids remaining after evaporation of a water sample).
Transmissivity - The rate at which water of a prevailing density and viscosity is transmitted through a unit width of porous medium under a unit hydraulic gradient. It is a function of properties of the liquid, the porous media, and the thickness of the unit and is the product of hydraulic conductivity and the saturated thickness of the aquifer (metres2/second).
Transpiration - Loss of water vapour from plants.
Trend - The relationship between a series of data points (e.g. Mann Kendall test for trend).