Yealm is a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in Dartmoor

Dartmoor

Dartmoor is an area of moorland in south Devon, England. Protected by National Park status, it covers .The granite upland dates from the Carboniferous period of geological history. The moorland is capped with many exposed granite hilltops known as tors, providing habitats for Dartmoor wildlife. The...

 in Devon

Devon

Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...

 in south-west England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

. It rises 1,411 feet above sea level on the Stall Moor mires of south Dartmoor and makes a 12 mile journey to the sea, passing through Cornwood

Cornwood

Cornwood is a village and civil parish in the South Hams in Devon, England. It has a population of 988.- Religion :The Church of St Michael's is Cornwood's parish church....

, Lee Mill and Yealmpton

Yealmpton

Yealmpton is a village in the English county of Devon. It is located on the A379 Plymouth to Kingsbridge road and is about from Plymouth. It derives its name from the River Yealm that flows through the village. Yealmpton is home to a 400 year-old stone cottage, where it is said, a version of the...

, before reaching the estuary mouth just below Newton Ferrers

Newton Ferrers

Newton Ferrers is a village in the civil parish of Newton and Noss in the English county of Devon, located about south-east of Plymouth on the River Yealm estuary. It lies within the South Devon Area of Outstanding Natural Beauty.-History:...

 and Noss Mayo

Noss Mayo

Noss Mayo is a village in south-west Devon, England, about 6 miles south-east of Plymouth. It lies about a mile inland, on the southern bank of Newton Creek, an arm of the estuary of the River Yealm. On the opposite, northern bank of the creek is Newton Ferrers, a slightly larger settlement. The...

.

This short river once supported relatively large stocks of salmon

Salmon

Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...

, sea trout and brown

Brown

Brown is a color term, denoting a range of composite colors produced by a mixture of orange, red, rose, or yellow with black or gray. The term is from Old English brún, in origin for any dusky or dark shade of color....

 trout but their populations have dwindled since the 1940s.

The Yealm estuary has a large oyster bed. The fertility of estuaries relates to the fact that when different water masses meet there is usually a bloom of the phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

, which can double its numbers/mass every 24 hours, but nude microscopic algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 multiply so quickly they can consume all the available plant nutrients until one nutrient is absent and they return to dormancy. But when two bodies of water meet, as they do in estuaries, it is unlikely that the same nutrient is missing in both. Here, the River Yealm and its estuary are well-matched, so the fresh and sea waters mix in approximately ideal proportions while there is always a reservoir of blooming mixed water that is not washed out to sea at low Spring tides. Some other South Devon estuaries do not share this feature: some, like Salcombe

Salcombe

Salcombe is a town in the South Hams district of Devon, south west England. The town is close to the mouth of the Kingsbridge Estuary, built mostly on the steep west side of the estuary and lies within the South Devon Area of Outstanding Natural Beauty...

, receive too little fresh-water while some, like the Dart

Dart

-Dart:In popular culture* Dart , an Image Comics superhero* Dart Feld, the central character of the PlayStation RPG, The Legend of Dragoon* Wraith dart, an attack spacecraft on Stargate Atlantis....

 receive too much, at least for ideal shellfish production.

Cockles and wild oysters can be found on the shore at low tide in the right season.