"mainstem" meaning in English

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "main",
        "3": "stem"
      },
      "expansion": "main + stem",
      "name": "compound"
    }
  ],
  "etymology_text": "main + stem",
  "forms": [
    {
      "form": "mainstems",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "mainstem (plural mainstems)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "2015 July 9, “Diverse Early Life-History Strategies in Migratory Amazonian Catfish: Implications for Conservation and Management”, in PLOS ONE, →DOI",
          "text": "Piramutaba (Brachyplatystoma vaillantii) make up a second large export fishery and are found almost exclusively in the Amazon River mainstem, whitewater tributaries, and the estuary [30 –32 ].",
          "type": "quotation"
        }
      ],
      "glosses": [
        "The principal watercourse of a river that has multiple streams"
      ],
      "id": "en-mainstem-en-noun-ibw2E096",
      "links": [
        [
          "watercourse",
          "watercourse"
        ],
        [
          "river",
          "river"
        ],
        [
          "stream",
          "stream"
        ]
      ]
    }
  ],
  "word": "mainstem"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "en",
        "2": "main",
        "3": "stem"
      },
      "expansion": "main + stem",
      "name": "compound"
    }
  ],
  "etymology_text": "main + stem",
  "forms": [
    {
      "form": "mainstems",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "mainstem (plural mainstems)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English compound terms",
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "2015 July 9, “Diverse Early Life-History Strategies in Migratory Amazonian Catfish: Implications for Conservation and Management”, in PLOS ONE, →DOI",
          "text": "Piramutaba (Brachyplatystoma vaillantii) make up a second large export fishery and are found almost exclusively in the Amazon River mainstem, whitewater tributaries, and the estuary [30 –32 ].",
          "type": "quotation"
        }
      ],
      "glosses": [
        "The principal watercourse of a river that has multiple streams"
      ],
      "links": [
        [
          "watercourse",
          "watercourse"
        ],
        [
          "river",
          "river"
        ],
        [
          "stream",
          "stream"
        ]
      ]
    }
  ],
  "word": "mainstem"
}