# nwis.streamflow.annual_stats

Get year-by-year annual mean streamflow statistics for a USGS gauge station. Returns mean discharge (ft3/s) for each water year with records going back to 1900s at major stations. Useful for long-term hydrologic trend analysis, climate change assessment, and water resource planning. Typical major stations have 60–90 years of annual records. Use water.sites to look up station numbers by state or bounding box. No auth — USGS public domain, unlimited calls.

Agent View of the PolicyLayer registry record for `nwis.streamflow.annual_stats`. HTML page: https://policylayer.com/tools/io-github-whiteknightonhorse-apibase/nwis.streamflow.annual-stats

## Facts

- Tool: `nwis.streamflow.annual_stats`
- Server: Apibase (`apibase-mcp-client`) — https://policylayer.com/tools/io-github-whiteknightonhorse-apibase.md
- Install: `npx -y apibase-mcp-client`
- Homepage: https://github.com/whiteknightonhorse/APIbase
- Risk category: Read (Low risk)
- Registry record: grade F, identity unverified
- Server auth posture: open
- Server rate-limited: no
- Parameters: 1 (1 required)
- Recommended policy verdict: Allowed

## Parameters

| Parameter | Type | Required | Description |
| --- | --- | --- | --- |
| `site_no` | string | yes | USGS gauge station number, 8–15 digits (e.g. "01646500" for Potomac River near DC). Use water.sites to find station numbers. |

Parameters from the server's own tool schema.

## Example call (MCP tools/call, JSON-RPC 2.0)

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "nwis.streamflow.annual_stats",
    "arguments": {
      "site_no": "<site_no>"
    }
  }
}
```

## Why nwis.streamflow.annual_stats is rated Low

Tool retrieves historical hydrologic data without modification, deletion, or external effects.

From the tool's own definition: "Get year-by-year annual mean streamflow statistics. Returns mean discharge. Public domain, unlimited calls."

## Use case

AI agents call nwis.streamflow.annual_stats to retrieve information from Apibase without modifying anything. It is typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.

## Recommended policy (PolicyLayer)

Verdict: **Allowed**. Enforced by the PolicyLayer MCP gateway (https://policylayer.com/mcp-gateway) before a call reaches Apibase:

```json
{
  "version": "1",
  "default": "deny",
  "tools": {
    "nwis.streamflow.annual_stats": {}
  }
}
```

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## For agents

This record is a snapshot. Live verdicts and the full registry:

- Check every server in your MCP config at once: `npx -y policylayer stack`
- Vet a server before you add it: install the mcp-precheck skill — `npx skills add https://policylayer.com` (skill text: https://policylayer.com/skill.md)
- Query the registry over MCP: endpoint `https://api.policylayer.com/mcp` — tools `check_mcp_server`, `check_mcp_stack`, `check_tool`, `search_registry`, `get_change_events`

---

Source: the PolicyLayer MCP registry — one continuously verified record per MCP server. Full record: https://policylayer.com/registry?q=io-github-whiteknightonhorse-apibase · API: https://policylayer.com/registry/api · Policy library: https://policylayer.com/policies/io-github-whiteknightonhorse-apibase