Arroyo de en Medio (Spanish for "in the middle creek") is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data
The National Map
, accessed March 15, 2011 coastal stream in western

San Mateo County, California San Mateo County ( ), officially the County of San Mateo, is a county located in the U.S. state of California. As of the 2020 census, the population was 764,442. Redwood City is the county seat, and the third most populated city following Dal ...

. Arroyo de en Medio rises on the western slopes of

Montara Mountain Montara Mountain, positioned between the unincorporated community of Montara, California, to the southwest and the city of Pacifica, California, to the north, forms the northern spur of the Santa Cruz Mountains, a narrow mountain range running the ...

and discharges to the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

Half Moon Bay Half Moon Bay is a coastal city in San Mateo County, California, United States, approximately south of San Francisco. Its population was 11,795 as of the 2020 census. Immediately at the north of Half Moon Bay is Pillar Point Harbor and the un ...

at the location of the unincorporated community of Miramar at Miramar Beach. The watershed of Arroyo de en Medio consists of relatively permeable sandy soils capable of significant recharge to its aquifers, which supply considerable potable water to the local area. ''Arroyo'' is Spanish for creek; ''de en Medio'' means literally "of in between" but a closer more functional translation may be ''in the middle''.

Groundwater

In the headwaters area, coarse grained

decomposed granite Decomposed granite is a kind of granite rock that is weathered to the point that the parent material readily fractures into smaller pieces of weaker rock. Further weathering yields material that easily crumbles into mixtures of gravel-sized par ...

overlies heavily fractured

granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...

based bedrock

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...

s. This upper pocket groundwater complex feeds down-basin

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. All ...

fan deposits on the

coastal plain A coastal plain is flat, low-lying land adjacent to a sea coast. A fall line commonly marks the border between a coastal plain and a piedmont area. Some of the largest coastal plains are in Alaska and the southeastern United States. The Gulf Co ...

. These alluvial fans exhibit some characteristics of a confined aquifer, but also respond to recharge from

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

Mouth

At the mouth of Arroyo de en Medio, the stream cuts through a flat marine terrace plain of alluvium, forming steep sided banks of approximately 40 degrees with respect to the channel floor; the channel floor is about eight meters deep relative to the bank tops. Even at the mouth, Arroyo de en Medio is considered intermittent, since the flow typically ceases in the summer months. However, there is some summer standing water in the channel near the mouth, the result of street runoff from summer irrigation and car washing. The stream mouth is clear of canopy vegetation but densely filled with

sedge The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" genus '' Carex'' ...

s in a

marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found a ...

y setting. The streambed at the mouth is essentially covered by water dependent plant species. These include

willow Willows, also called sallows and osiers, from the genus ''Salix'', comprise around 400 speciesMabberley, D.J. 1997. The Plant Book, Cambridge University Press #2: Cambridge. of typically deciduous trees and shrubs, found primarily on moist so ...

(''Salix'' sp.),

(''

Carex ''Carex'' is a vast genus of more than 2,000 species of grass-like plants in the family Cyperaceae, commonly known as sedges (or seg, in older books). Other members of the family Cyperaceae are also called sedges, however those of genus ''Carex'' ...

'' sp.), stream

monkey flower Erythranthe, Monkey flower can refer to: *Several genera of plant family Phrymaceae, including: ** ''Diplacus'' ** ''Erythranthe'' ** ''Mimulus'' *Various snapdragon-like Lamiales, including: ** ''Linaria vulgaris'' ** ''Phyllocarpus septentrionali ...

(''Mimulus'' sp.) and

red alder ''Alnus rubra'', the red alder, is a deciduous broadleaf tree native to western North America (Alaska, Yukon, British Columbia, Washington, Oregon, California, Idaho and Montana). Description Red alder is the largest species of alder in Nort ...

(''

Alnus oregona Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...

''). These and other riparian plants form the food resource of many

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''Biota (ecology ...

l species, either directly or indirectly. Many orb web weaving spiders at the mouth indicates the abundance of insects utilized as prey by certain

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

s such as

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

tree frog A tree frog (or treefrog) is any species of frog that spends a major portion of its lifespan in trees, known as an arboreal state. Several lineages of frogs among the Neobatrachia have given rise to treefrogs, although they are not closely rela ...

s, lizards, and many resident and migratory birds. The site is also utilized for

avafauna Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight s ...

l nesting and breeding. In the 1989 survey by Earth Metrics, no sightings of the

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...

San Francisco garter snake The San Francisco garter snake (''Thamnophis sirtalis tetrataenia'') is a slender multi-colored subspecies of the common garter snake. Designated as an endangered subspecies since the year 1967, it is endemic to San Mateo County and the extrem ...

were made, although this snake was observed at nearby

Denniston Creek Denniston Creek is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map , accessed March 15, 2011 coastal stream in western San Mateo County, California. Denniston Creek rises on the western slopes of ...

. The mouth reach is classified as a

riparian A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...

corridor under the strict meaning defined by the San Mateo County Local Coastal Plan

References

{{reflist Rivers of San Mateo County, California Rivers of Northern California

Groundwater

Mouth

See also

References