As rain falls around us

by Brian T. Lynch, MSW

• Oceans hold 97% of all Earth's water, which is salty.

• Every drop of rain is freshwater that is distilled and pure, thanks to the rain cycle. 
 • About 3% of the Earth’s water is fresh, two-thirds of which is trapped in ice or permafrost. 
• About half of all rainwater that falls on land returns to the air due to evaporation or transpiration that results from photosynthesis (essentially, plant breath). 
 • Gravity draws most of the other half into river basins and out to sea. 
• Every roadway storm drain is a tributary to a local stream within a river basin. 
• A fraction of all rainfall filters underground to fill aquifers (underground reservoirs). 
• The average rate that rain travels down to the aquifers is 10 feet per year. 
• Under every river or freshwater waterbody is a wider, deeper flow of groundwater heading to the sea: 
   -  When stream banks are full, some water filters into stream banks, which increases groundwater reserves. 
   -  When stream flow is low, groundwater seeps back into the streams, keeping water volume more stable and preserving aquatic habitats. o When air temperature is hot, cool groundwater helps keep stream temperatures cooler. 
  -  When air temperatures are cold, groundwater keeps water temperatures warmer, making streams and rivers more habitable. 
• Riffles (rocky bottom areas) and rapids add oxygen to streams and rivers, • Cooler water holds more essential oxygen than warmer water. 
• Vegetation along streams and riverbanks provides nutrients for aquatic life, keeps the water cooler, and provides essential habitat.  
• The hyporheic, or “liver of the river,” is a subsurface layer where groundwater and surface water meet. It serves an important filtration function: 
   -  A transition zone between two larger ecosystems where species from both zones mingle and unique hyporheic organisms exist. 
   -  It can be dozens of feet thick and many miles wider than the river.     -  Water flows through the hyporheic layer in the same direction as the surface water but at much slower rates. 
   -  The upward flow of groundwater in this zone brings oxygen needed for fish eggs to develop. o In healthy streams, this zone is full of life. Crustaceans, worms, and aquatic insects move between it and the stream bottom. 
   -  Nematodes, copepods, rotifers, and tardigrades burrow throughout the zone, creating pathways that allow groundwater and surface water to mix. 
  -  Microbial life in the zone metabolizes inorganic compounds to create food for other tiny organisms. It functions like our gut bacteria, detoxifying certain harmful chemicals while producing other compounds essential for good health.