Question: What is the river that Sangin is found in the valley of the primary watershed for?

Context:
Helmand River
The Helmand River (also spelled "Helmend, Helmund, Hirmand"; Pashto/Persian: هیرمند, هلمند "Hīrmand, Helmand ", Greek: "Ἐτύμανδρος " ("Etýmandros"), Latin: "Erymandrus ") is the longest river in Afghanistan and the primary watershed for the endorheic Sistan Basin.
Sonoma Creek
Sonoma Creek is a 33.4 mi stream in northern California.
 It is one of two principal drainages of southern Sonoma County, California, with headwaters rising in the rugged hills of Sugarloaf Ridge State Park and discharging to San Pablo Bay, the northern arm of San Francisco Bay.
 The watershed drained by Sonoma Creek is roughly equivalent to the wine region of Sonoma Valley, an area of about 170 sqmi .
 The State of California has designated the Sonoma Creek watershed as a “Critical Coastal Water Resource”.
 To the east of this generally rectangular watershed is the Napa River watershed, and to the west are the Petaluma River and Tolay Creek watersheds.
Guadalupe Watershed
The Guadalupe Watershed consists of 170 sqmi of land within northern California's Santa Clara County.
 This watershed is owned and managed by the Santa Clara Valley Water District.
 The surface runoff from this area drains into the various rivers (namely the Guadalupe), streams, reservoirs or other bodies of water which all eventually gets carried into the San Francisco Bay (indicated below, with surrounding counties in red).
 Essentially, all the water from the creeks and rivers that make up the Guadalupe watershed, including water from storm drains, flows into the Guadalupe River, and then flows downstream into the San Francisco Bay at the Alviso Slough in Alviso.
 The Guadalupe watershed's main tributaries include Los Gatos Creek, Trout Creek, Hendrys Creek, Ross Creek, Pheasant Creek, Rincon Creek, Herbert Creek, and Golf Creek.
 Six major reservoirs exist in the watershed: Calero Reservoir on Arroyo Calero, Guadalupe Reservoir on Guadalupe Creek, Almaden Reservoir on Los Alamitos Creek, Vasona Reservoir, Lexington Reservoir, and Lake Elsman on Los Gatos Creek.
Rideau Valley Conservation Authority
The Rideau Valley Conservation Authority (RVCA) is an inter-municipal environmental protection and advisory agency that works with local municipalities, government agencies, special interest groups and the general public to protect the natural resources of the Rideau River watershed.
 The watershed drains an area of over 4,000 square kilometres of Eastern Ontario and includes towns such as Portland, Perth, Smith Falls, Merrickville, Kemptville, and Manotick.
 About 620,000 live in the watershed, mostly deriving from the City of Ottawa.
 There are 18 municipalities within the valley and most people, outside of Ottawa, draw their drinking water from the river or from groundwater.
Mokhotlong District
Mokhotlong district (in Sesotho language "the place of the bald ibis") is a district of Lesotho.
 It includes the highest terrain in the Maloti Range and the source of the Senqu River, Lesotho’s primary watershed.
 Mokhotlong is the capital or camptown, and only town in the district.
 Mokhotlong borders on the KwaZulu-Natal Province of South Africa, with its north point bordering the Free State Province.
 Domestically, it borders on Butha-Buthe District in northwest, Leribe District in west, and Thaba-Tseka District in the south.
Baganga Protected Landscape
The Baganga Protected Landscape, in Mindanao in the Philippines, is one of four protected watershed areas of the province of Davao Oriental.
 It covers the Upper Baganga River Basin in the Mindanao Pacific Cordillera, the primary source of water for the Baganga Water District.
 The Baganga River, the main river channel of the municipality of Baganga, flows on a northeastern direction towards the Baganga Bay.
 The river system includes the Languyon River, Daquit River, Mahanob River, Dapnan River and Kinablang River all emptying into the Philippine Sea.
 It was established in 1987 as the Baganga Watershed Forest Reserve with an area of 114 ha .
 The watershed was declared a protected landscape in 2000.
Cañada del Oro
The Cañada del Oro (Spanish for "Canyon of Gold"), is a primary watershed channel in the valley of Tucson, Arizona, USA.
 The word "cañada" has a tilde "(ñ)" and is pronounced ] in Spanish; in English it is pronounced , not like the country of Canada.
Drangiana
Drangiana or Zarangiana (Greek: Δραγγιανή , "Drangianē"; also attested in Old Western Iranian as "Zranka" was a historical region and administrative division of the Achaemenid Empire.
 This region comprises territory around Hamun Lake, wetlands in endorheic Sistan Basin on the Iran-Afghan border, and its primary watershed Helmand river in what is nowadays southwestern region of Afghanistan.
Sangin
Sangin (Pashto: سنگين‎ ) is a town in Helmand province of Afghanistan, with population of approximately 14,000 people.
 It is located on in the valley of the Helmand River at 888 m altitude, 95 km to the north-east of Lashkargah.
 Sangin is notorious as one of the central locations of the opium trade in the south of the country, and is also a town that has traditionally supported the Taliban.
 It was described by British newspaper "The Guardian" as "the deadliest area in Afghanistan".
 Sangin also houses the main bazaar for Sangin District.
 Route 611 passes through Sangin.
Milliken Creek (California)
Milliken Creek is an 11.9 mi stream in Napa County, California that is tributary to the Napa River.
 There are approximately 7300 acre in this watershed, of which 90 acre are developed as vineyards.
 Milliken Creek rises on the western slopes of the east side of the Napa Valley and flows through the Silverado Country Club property.
 Much of this watershed property had once been part of the Mexican land grant Rancho Yajome, which had been granted to General Mariano G. Vallejo.
 Most of this watershed was wilderness area to at least 1869, and thereafter the lower watershed was begun to be developed as pasture and grazing agricultural uses.
 In an 1989 stream survey by Earth Metrics, the steelhead fishery was found to be robust up to and including the Silverado Country Club.

Answer: Sistan Basin