The answer is conservation. Hope that helps!
Well, first off, the sediments get brought down from the mountains after eroding away. Those sediments combine with others as it passes through the creeks and streams, rivers etc. Whatever dies, for example lets say chum salmon since they have nutrients within them. They aren't good swimmers so they are normally at the mouth of the river or in the small streams nearby. When they die, their nutrients go into the soil, that soil or whatever nutrients in that, could be swept out to the wetlands and piled up. Thus, adding rich nutrients. Adding the sediments from high up, you have a rich supply of nutrients from the wildlife that dies there as well.
The water from a trough and the temperature outside would be abiotic factors. the grass it grazes on or a cowbird picking bugs off of it would be biotic factors (for future reference, abiotic factors are caused by nonliving things, and biotic factors are caused by other organisms)
It cant make any more cell, so ya it dies...
