The first analogy relates fertilizer run-off with depletion of oxygen. This relationship is actually the effect of fertilizer run-off on oxygen. When nutrients (such as phosphorus and nitrogen) which are supplied by fertilizers that plants need to grow well, find their ways into water bodies such as lakes and groundwater, this situation is called fertilizer run-off. When the nutrients get into the water bodies, they bloom algae present in the water and the more the bloom, the more the depletion of oxygen. In other words, fertilizer run-off causes depletion of oxygen.
Using the same analogy, delta formation and low water flow have the same relationship. In other words, the effect of delta formation is that it leads to low flow of water. Delta formation occurs when a river carries sediments as it flows towards another water body (which is usually not flowing well or not flowing at all) and drops the sediments at the mouth of this stagnant water that has no power to carry the sediment so much that these sediments build up to form layers called delta. When the delta is formed, there is a reduction in the flow of water.
