The Alpheios (, , Latin Alpheus), sometimes spelled Alfeiós, is the main stream of the Alpheios Valley drainage system, a dendritic type, originating on the north slopes of Mount Taygetus, located in the center of the Peloponnesus of Greece, and flowing to the northwest to the vicinity of Olympia, where it turns to the west and, after being impounded by the Flokas Dam, a hydroelectric facility, empties into the Gulf of Kyparissia of the Ionian Sea south of Pyrgos. The entrance into the gulf through agricultural land and across an unpopulated, sandy beach partially blocked by a spit is hydrologically unspectacular, with the water too shallow to be navigable by any but the smallest craft.
The concept of a single source has little meaning for most of the rivers of Greece, which begin as a confluence of multiple springs in the mountain valleys. There is almost never just one, although most may be unreported or neglected. Thus it is appropriate to speak "a source" or "the sources" but never "the source."
Nevertheless, competing villages sometimes claim to own "the source." Moreover, the sources are not geologically stable, but change frequently in history. In karst terrain, such as the Peloponnesus, the population is acutely aware that rivers may run underground for some distance. Thus the "source" of the Alpheios has always been a subject of debate and literary fantasies, some wild by modern geologic standards. Most recently attempts have been made to connect the Alpheios through underground channels to the "40 rivers" region of the high plateau of central Arcadia (around Tegea and Mantineia, etc.).
The Alpheios of today bears little resemblance to the historical Alpheios. Much of it has been widened by damming; large sections have been straightened by embankments; flood control works have been constructed; water for municipal use and irrigation is diverted all along the course; some sections are used for gravel mining; and waste water, fertilizer, and pesticides pollute it from one end to the other. It has been necessary to establish regular monitoring by the government and create substructure to make important decisions concerning the fate of the river.
Despite the recent alterations of man and the apparently random dendritic pattern in parts of the valley, a geologic pattern emerges that is too regular to be entirely random. The valley is a regular trough, or basin, from the coast to the interior. The dendrism is mainly right-bank. On the left is the long ridge of Mount Lykaion, relatively uneroded, suggesting that different types of rock occur on either side. After a divide the ridge continues as Mount Taygetus, and the valley as the Eurotas River valley, which extends south to the Gulf of Lakonia. The entire southwest Peloponnesus is split from Arcadia by this great trough, considered two basins, which must have been in place in some form before any extensive dendrism.
Scarps along the trough suggest that the two basins are rift valleys, depressions, or graben, caused by the ridges on each side moving away from each other. The whole, finger-like arrangement of the ridges in the south Peloponnesus is currently explained as a stretching, or extension, of the Peloponnesus in a NE-SW direction, pulling the troughs apart. The division between the ridges occurs because they were primordially compressed together in the Hellenic orogeny, becoming different modern zones of rock. Subsequent to the compressional regime back-arc extension of the Hellenic arc, the outer ring of islands, opened the Aegean Sea and initiated the extensional regime that is opening the Peloponnesus.
Fortuitously the right bank of the Alpheios Valley became a defensive zone for the protection of central Arcadia. During the so-called Dorian invasion starting about 1000 BC iron-age highlanders (Dorians) from central Greece overran the bronze-age coastal regions of the Peloponnesus, expelling or subjugating the Mycenaean population there, but could not ascend the right-bank mountains to take central Arcadia. The population there maintained its culture (Arcado-Cypriot) and political independence. The Dorians kept the valley, which accounts for the predominance of Doric architecture at Olympia.
Hydrologic characteristics Classification of the Alpheios In their 2003 master plan for the management of Greek water resources, the then Hellenic Ministry of Development, one of the many Greek governmental agencies that have been responsible for the monitoring and care of Greek rivers, noted that of the 765 recorded rivers in Greece, 45 were permanently flowing, of which the Alpheios is one. Although the Greek government had been monitoring some rivers since the 1970s, classification and monitoring of rivers was not mandated by the European Union until the Water Framework Directive (WFD) of 2000. The EU was interested in identifying quality problems with the waters of Europe and rescuing degraded waters. The Hellenic Center for Marine Research (HCMR) therefore added the study of water quality in Greece to its previously existing AQEM program. Select river sites were monitored for three seasons of 2000–2001, spring, summer, and winter, the results appearing in an online report of 2005.
Classification by catchment area Several figures have been reported for the catchment area of the Alpheios. A 2004 study by the Civil Engineering Department of the University of Patras reported , and a 2020 study by a consortium of academic departments including the HCMR reported . The value has been reported by the Greek Ministry of Environment, Energy and Climate Change. A research group of the National Technical University of Athens found the same area, within (but not equal to) the rectangle and . The figure depicts a catchment outlined in black with the tributaries shown in detail and a terrain map color coding the altitudes. By HCMR standards, this makes the Alpheios a large river (over 1000 km2. basin). The uppermost part of the Alpheios basin, at Karytaina, upstream from its confluence with the Lousios, is .
The maximum elevation is with a mean elevation of . The mean slope is about 14 deg. The catchment is about 25% of the map area of the Peloponnesus.
The major part of the basin is mountainous, with 52.5% of the catchment map being over 600 m. The rest is hilly and flat along the coast (10.5%). The political distribution is 60% Arcadia, 30% Elis, and 10% Achaea. Arcadia accounts for most of the mountainous and hilly terrain. Achaea shares some of the mountainous terrain to the north, while Elis contains the lowlands and some of the hilly terrain.
Classification by zone The 2006 HCMR article defines three river zones of Greece, a "zone" being a region of a similar geophysical terrain, which influences the chemical composition and physical morphology of the rivers in it. Zones 1, 2, and 3, are based on the geologic isopic ("same facies") zones, or tectono-stratigraphic units of Greece, which are ridges of distinct rock trending northwest to southeast.. It is theorized that these ridges are their own types of rock because they represent separate microcontinents that migrated across Tethys Sea from Gondwana and docked against Eurasia during the compressional regime of the Hellenic orogeny, a branch of the Alpine orogeny, when parts of the African plate were being pushed against the Eurasian plate and being subducted under it.
The isotopic zones are a product of the Continental drift theory and its modern development, plate tectonics, which prevailed after 1958. The earliest ideas of the Hellenic orogeny are a product of the previous Geosyncline theory, in which the mountains of the Earth were believed to have been raised by compression of large depressions, or geosynclines, caused by shrinkage, and infilled with sediment. It was hypothesized that the mountains of Greece, called the Hellenides, arose from a Hellenic geosyncline in two phases, a volcanic breakthrough in the center, resulting in the "inner Hellenides," and a subsequent orogeny at the west periphery from the wrinkling of sediment running off the central mountains. The peripheral range was the "outer Hellenides." As both theories featured a compressional regime, the Hellenides terminology was kept, but it had to be matched to the isopic zones.
The Alpheios falls within river zone 3, the Outer Hellenides, an arc-like range continuing the Dinarides of the Balkans into western Greece, comprising most of the Peloponnesus, and including the southernmost arc of Aegean islands, with Crete, around to the coast of Anatolia (Hellenic arc). The isopic zones included, west to east, are The Ionian, the Tripolitsa, the Pindos, and the Parnassos. To the east are the inner Hellenides, partly volcanic, with the dividing line down the center of Greece. They end at the Chalcidic Peninsula, beyond which are the Rhodope and Serbomacedonian massifs.
River morphology The question of the source Mathematical hydrology requires definite numbers: rivers have lengths, cross-sections, measurable linear and volumetric (discharge) rates of flow. The length of a river requires a source. If the Alpheios is a permanently flowing river, then there must be a place of higher elevation where the flow begins, and that is by definition the source. Water comes from a multiplicity of sources. A definite river, however, must have a definite source. The distance along the river from mouth to source is the length.
Lengths, however, like catchments, are not geodetic. They do not take the curvature of the Earth into consideration nor the elevations of the terrain. For example, seen from above, a waterfall is only a few feet of river length, yet the stream may fall hundreds of uncounted feet. Map work for measuring the general morphology of rivers relies on the conformal map, pieced together from satellite photographs and topographic maps.
The length of a river by any system of measurement is not a constant either of measurement or of nature. The measurer working his way along the river encounters many points requiring a decision. For example, the Alpheios divides itself around midstream islands at many points. The river distance through the island zone depends on the stream chosen or the average decided upon. If the river has been impounded or widened by the works of man, the estimator must decide which course through the impoundment applies. The length also depends on the method of estimation. Representation of the river by small straight-line segments depends entirely on the segmentation decisions made. Nature itself does not provide reliable and exact lengths. Rivers change course in history (let alone geologic history) to such a degree that they are typically different from century to century, as the geology of erosion teaches they must be.
The publication sources therefore give riverine lengths and locations that are different from each other, unless one source copies another. The length can be assumed to have been measured at least one or more years before publication, many years if one source relies on another. Some lengths are , , , . Encyclopædia Britannica uses 110 km with a stock phrase "the largest of the Peloponnesus," paraphrased by most of the sources, without definition or evidence. These lengths, representing the general trend, aim at a single location, about due south of Megalopoli, where the topographic maps show the beginning of a single stream. Megalopoli is situated on a divide between the Alpheios and its tributary, the Elisson, at about . The start of the single stream is at about .Otherwise unreferenced altitudes and distances were estimated or taken from topographical maps, mainly of Google Maps, or calculated using the Google distance estimator.